[RPG] spell, item, or other game feature that would give the character insight to an enethe’s weaknesses and resistance

You've already done what you can for that particular situation

The rest is up to those two.

I tried to tell him not take it seriously and that he acted was wrong but he tuned me out. I know that the fighter feels terrible because he, and everyone playing, didn't realize how important the last kill was to him. So before I left I told him that I understand that he is angry but he should call the fighter. I explained that maybe if he express his feelings then the two of them could comprise or come to an understanding.

If those two cannot, after time to settle down, make peace with each other then their time together in the party is going to be short. D&D 5e is generally a team game. It is a game of taking turns (basic stuff from kindergarden) and sharing in team success.

I plan to call both the fighter and other player to see how they are feeling and their concerns on the matter.

Since most of the advice on this site about player conflict is "talk to the players" you are already on the right path. You may be wondering about what the next few sessions will hold for you, since you want this to be fun and not interpersonal drama.

What can I as a DM do to prevent problems like this?

1. Make sure everyone gets their turn. (In a mechanical sense, stick to the initiative order, and make sure that the players know when it is their turn).

   • @umbranus made this point in a comment:

     ... I suggest using initiative to solve stuff like that. The Fighter used his turn to throw the barrel, so everyone else should get a turn before the Fighter goes again¹.

2. Make sure each player gets some time in the spotlight.

3. Remind everyone that we play these games for fun.

The link to the kill steal problem that @DaleM provided is worth reviewing before your next session.

¹ If the reason that the Rogue was unable to take his action is that you gave the Fighter a turn (tossing the barrel) and then he got to take his chance before the Rogue did, then there's some DM fault in this for not keeping the initiative/turn order going. However, if they were all over the room and the Fighter was closer, or was able to get there first in the regular turn order, then you have a different problem to deal with, as above.

I think the intent is clear from how the two work together. The froghemoth takes half damage from lightning, but on that turn the froghemoth has a hit to its AC and speed. My guess at the author's intent is that doing all that other stuff and taking the full amount of damage would have made the monster too weak, so halving damage allowed them to add the interesting effects.

Moreover, in previous editions (3.5 at the very least), the froghemoth was immune to lightning damage, but would instead take a speed hit. As far as 5e goes, I don't recall any other creatures that have both a similar resistance plus other negative effects from the same damage type.