One, don’t “stand up for” anybody without talking to them first

It’s possible that this is someone who doesn’t know how to stand up for herself, and will appreciate the assistance. Far more likely, however, it will just humiliate and anger her—“standing up for her” without talking to her first is condescending (implies she cannot manage for herself) and incredibly disrespectful (stealing her of her agency and right to decide how to manage her own affairs). That is how a child is treated, and then usually only by a parent. Don’t treat a friend that way.

So ask her if this has been bothering her—you say she seemed to be on the verge of tears, so you certainly think so, but don’t assume. You could have misread the situation, maybe (I don’t know you or how good a judge of “on the verge of tears” you are), but even if you didn’t, for all you know she could be upset about something else, and anyway she may not appreciate knowing that her emotions were “on her sleeve,” so to speak. Getting upset and emotional about things can be embarrassing—it sounds like an upsetting situation, so she should have nothing to be embarrassed about, but nevertheless most people would be.

Ultimately, this is up to her. If she doesn’t want a confrontation—and since this is about her character, she would be pulled into any confrontation you started—you need to respect that. There are alternatives here; for example, eldritch blast is generally a much-superior warlock spell over toll the dead, and since she rolls the attack roll you can all see it. The DM can still bluff about the target’s AC, but since presumably she is far from the only person making attack rolls, he would have to be consistent about that. If she rolls a 24 and misses, and the next person rolls an 18 and hits, that’s pretty blatant. (And if the DM does get that blatant, she may become more comfortable with confrontation.)

In short, if you want to support her, you must support her, which means abiding by any decisions she makes here.

Second, “calling him out” is fairly unlikely to get useful results

That sounds likely to just make someone defensive; if he doesn’t deny it and defend himself, he may feel as though he is admitting to cheating—even if he is, he very likely doesn’t want anyone to know it.

And more to the point, you don’t know that he is cheating. The math, as Xirema’s excellent answer discusses, certainly suggests that something is wrong, but “cheating” is only one possible answer out of many. If nothing else, Hanlon’s razor easily applies here: it may very well be that he doesn’t realize that he’s making some serious mathematical error here. Treating a 3.5e creature’s Will saving throw as its Wisdom saving throw would certainly do it, for example.

If you ignore this, and go straight for accusations, your best case scenario is that the group decides you’re right, the DM is lying, and they stop playing with him. If someone else from the group is willing to DM, then you can maybe have a game without him. But that’s a lot of if, and it would be massively disruptive—and that is the best case scenario. The worst case scenario is something like humiliating the person you were trying to help, alienating your friends, and becoming uninvited from the game yourself.

Third, there may not be good outcomes available here

You are talking about someone who, you suspect, cheated and as a result (the intended result?) pushed one of his players “to the verge of tears.” That is a rather extreme abuse of the DM position. If it was merely a mistake—hope it was!—and he’s willing to cop to that and fix things—hope he is!—then all may be well. If not, though, you have a severe interpersonal problem here. If you are correct in your suspicions, you may very well want to strongly consider you stopping your play with him. His behavior towards your fellow player has clearly bothered you. And even beyond that, if he is willing to treat one player like this, how long until you draw that ire? Not long, is my guess, once you start questioning it.

And that might be the best way for you to support her—if you offer her the solidarity of also being willing to leave the game over this, so she isn’t alone in being bothered by it, that may make her feel better, may improve her willingness to leave herself. It may impress upon your fellow players, upon the DM, that this isn’t acceptable. It may be that the entire group leaves—that is, that the DM is effectively booted by having his players no longer willing to play under him.

Again, this is a pretty dire result, and may again be a best case scenario. Care is required here. Your fellow player has reason to be reluctant to get into this confrontation.

Fourth, I apologize but it must be said—don’t be a creep

First, please don’t take this personally. This isn’t even directed at you, per se—answers here are written for all who have an interest in the question and read them. So this is to anyone who finds themselves in a similar situation: be very, very careful here. Your user icon is a male face; your question uses female pronouns for your fellow player, so I presume you are male and your fellow player is female. Ultimately, that shouldn’t matter, and it doesn’t to an extent—you could be gay, or a woman who just chose to use Dr. Horrible for an icon, or she could be male despite your pronouns (or another reader could be in a similar situation except the one being bullied is male), and all of the below would still be a concern. It’s just, unfortunately, a concern with a greater likelihood in the case of you being a heterosexual male and she being female.

There is a long, ugly history of men who “stand up for” a woman—particularly without her expressing any desire for that to happen—and then thinking that this action has changed something about their relationship. It doesn’t: you are, as you were, friends. Anything you are doing here, you are doing because she is a friend and because your DM’s behavior bothered you, too. Any expectation you might have of her as a result—even simple gratitude—is inappropriate. She may not be grateful—she may very well ask you to mind your own business, or be mortified and furious if you don’t give her the chance to ask that. Even if she accepts your assistance, it must be offered in good faith and with zero expectation of reward.

And the odds are, unfortunately, rather good that these concerns are going to jump to the forefront of her mind when you bring this up. This behavior is extremely common. Women are, disgustingly, subjected to it with distressing frequency. They often have entirely too much practice in being wary about this kind of behavior.

You can’t change that, at least not immediately, by yourself, for this woman. You can’t change her past or experiences, if she like many has them. (Absolutely do what you can to push for societal change that makes this behavior less common!) So you have to go into this knowing that she may very well be wary that you have an ulterior motive. You have to respect that—you have to know why she has those suspicions, understand that it’s really not anything personal, just bitter experience. If you cannot accept her suspicions, and rise above begrudging her them, this is probably a bad idea to get involved in. You not only have to be respectful enough to genuinely want to help her just because seeing her treated in this fashion upsets you and you want it to stop, and you also have to be respectful enough to understand that she may not entirely trust that you are being as respectful as you are. If you lack the maturity or discipline to do that, you should very strongly consider dramatically scaling back your plans. Maybe it would be better to just offer some sympathy for a frustrating game and suggest hope that things will improve.