Should I allow a player to use wish to change a cantrips function?

Is it in any way gamebreaking, if I let a player use his wish to change a function of the green flame cantrip?

As a group of long year friends, we ruled that we always speak about the things the DM rules and talk about if it was right or acceptable for the players. I would never let the player speak his wish and then surprise him with an unforeseen bad consequence. We ruled that we talk about the wish and what would probably happen, if someone wishes that.

I just don’t know if it’s gamebreaking, if we allow it.

For example the green flame blade. The player wishes that the damage for the second target can also be dealt to the first target without the need of a second target in the first place. That means double damage output for a single target. But in the end the damage stays the same, except that it hits one instead of 2 targets.

He could word it like: I wish that all the power of my green blade spell directs only to a single target. But like I said I always talk with the player and listen to their view on this subject. So I don’t care about the wording. We allow it or we don’t.