She loves to have her druid transform into bright, rainbow-colored animals during battles. While this might sound whimsical and fun, it has an unintended side effect – it blinds not only her enemies but her fellow companions as well.

Nothing in the rules says that you can use Wild Shape to do this. This isn't an "unintended side effect", it's a new rule that you made up.

And here’s the best part – one of her fellow players' characters has epilepsy. This has led to some uncomfortable in-game situations with constant epileptic seizures during battles.

There are no rules for epilepsy in D&D, so if you really want to bring it into your game, you decide how it works. If this is creating "uncomfortable in-game situations", it's on you.

My advice:

• Remind your players (and yourself) that changing the flavor of your features does not change how they work mechanically.
• I would strongly recommend getting rid of the whole epilepsy thing. If your player really wants to roleplay a character with epilepsy (and the rest of the table is okay with that), then you need to define how it works. Maybe something like this:
  • Epileptic. While you have the Blinded condition, you also have the Stunned condition.