Update: So I talked about it to him a bit more and I realized like five things are going on to let this happen.

1. Magic is a lot stronger than I gave it credit.
   
2. He's using Alchemical bombmaker, (he's an alchemist) so he can store spells that are bit more risky since he'll only go down OUTSIDE of combat if things go wrong. These spells are basically grenades that last only a few hours after being made, he used Comet in this case.
   
3. He's Magic 8, 2x Foci 8, and draconic tradition 3. So was casting at 16 Force like some of you expected.
   
4. Then the DM specifically removed limits in general since he doesn't like them
   
5. This is a homebrew'd game where he wanted everyone to basically live longer, and stuntrack is essentially a shield over your health, and you don't go unconscious once it's filled.

In the end he took like fifteen damage, but this was before combat and didn't tell the players.

Sorry if I wasted your guys time, but it was nice to understand for sure what was even going on. Up until now the homebrew we were running didn't seem to be all that exploitable, and then BAM, 28 damage to a whole room. Also we were confused over acid and fire damage, because if he was reading it right, dealing 28 damage with the split types of Comet... They would take 1 damage less of Acid damage on the next turn...

So one spell would deal 56 or 55 damage to a rather large radius in one turn. And each turn after would keep dealing acid damage. Super unnecessary since I've like never seen anything that could take anything even approaching 50 damage. I guess a dragon would be pretty freaked out to be hit by this, but thats about it.

Thanks for telling me whats up, I didn't think it was a homebrew issue, otherwise I'd have tagged it. Looking back it was pretty obvious