If they're not gaining significant bonuses from the fire keyword, it's not going to break anything at all.
If they're a maxed out uberpyromancer: make them choose a power that actually is Fire (or use a magic item/feat that changes it to fire).
Damage types on their own are mostly even (although Radiant is the best, as so many undead are weak to it, and Poison is the worst as so many things are immune); it's when you start adding in feats that it becomes significant.
So, if they're doing it just for the flavour let them: it won't break anything.
If they're doing it because they get bonuses for that damage type: make them pay for the boost. They can use Flaming weapons as implements (staves can be flaming, as can swords/daggers) if the power was originally untyped or take an arcane admixture feat to change that one power.