The goal of the tactics below is to inform the GM what behaviours produce a good enjoyable game for all (point 2), what behaviours they should adjust (point 3), as well as (point 1) to try to make them aware of the fact that they have a different role at the table as the GM (as judge, as arbiter of fair, and as the interpreter and controller or rules) in addition to the role they had as a player (to have fun, to try to not ruin the fun of others)

It's unclear from the description if this person is new to GMing or not. This may be a case of the person GMing in the only manner they know how to play. This will be easier if that is the case, but even if not, my approach in this scenario is to use all of the following:

1) "Catch him in the act" ... When an attack roll that used to miss now hits, ask why. Accept the answer, but make sure to question every time. The idea here is to make the GM know that they are expected to play by rules (for the purpose of asking that the GM follow rules, what actual rules doesn't matter, just that they need to play by some rules).

2) Reinforcing (positive) Feedback ... When there is an encounter that runs right, talk it up. Say how fun it was. Say how tense it was. Point out swings that appear to happen naturally and how they produce story. The goal here is obvious - to encourage more of the good stuff.

3) Adjusting Feedback ... (which it sounds like you are at least attempting) When things are not right, tell the GM - after the fact is fine - you're not out to embarrass or insult, but to get the GM to be better next session - but make sure to say both why the encounter was not as fun as it could have been (the on-the-fly adjustments made it feel like deus ex machina, rather than real tension, with a real chance of failure) AND most importantly, what would have made it better (if the uber-whip-dude had been a little less tough, the start of the fight would have been easier, but the end of the fight would have been a tense touch and go affair)

Assuming you told him your thoughts...

You've Already Done It

And the "it" is to not play with this person anymore

You've talked to them. You agree on some points and not others. It sounds like this person's play style doesn't match yours. That's fine, happens to everyone. I consider myself pretty flexible when it comes to play styles and there's still people who I can't play with. They aren't bad people, the just expect things I don't.

Quit playing with this person.

If you haven't talked with them fully..

Talk With Them Fully

Lay bare your feelings on the matter. If both of your aren't willing to work together to come to a happy medium then you need to say goodbye. You do what you should do an any social situation where there is discord.

Now, my final point...

Why did you cancel your game?

It sounds like it's an issue with a single person, not the entire group. Do group or friendship dynamics keep you from running your game? Are you jaded because of the experience? If you were having fun without them, just send their character to the void and keep on going.