While I think the party might not be optimally built, it really shouldn't matter. I thought 5th Edition was built on the idea that you didn't have to have specialized builds to be competitive?
I actually prefer non-optimized characters. Giving that fighter a 15 Charisma because that's how you envision him, going with
lightning bolt over
fireball because you like the way it looks, or you are lightning-themed (with no mechanical benefit), etc.
However, even I draw the line. And a warlock using
fire bolt instead of
eldritch blast + Agonizing Blast is way over that line. If a player wanted to do that I would explain what the results would be an why that might be desirable. Most of the time that is all it takes. Once they see the difference it makes they are probably going to agree and change their option. If they really seemed to want to stick with it, I would
privately ask the other players how they felt about it (since something that sub-optimal affects the whole party). I wouldn't ask them in the presence of the warlock player because most people wouldn't want to rudely say that they don't want the warlock playing that way. If everyone was cool with the idea, then it's fine. If people aren't okay with it, I'd talk with the warlock player a bit more. Maybe it is just a matter of visuals.
Eldritch blast can look like fire. Or if they really insist it must
be fire, then maybe I'd give them a variant cantrip that is
eldritch blast but with fire instead of force damage.