One of my players, an assassin rogue recently took the Fey Touched feat and used it to learn Charm Person. In our last combat, they used it to successfully charm an enemy in the middle of the fight, asked the rest of the party not to attack them, tranquilly walked up to them, and then attacked.
So Charm Person says this:
If it fails the saving throw, it is charmed by you until the spell ends or until you or your companions do anything harmful to it. The charmed creature regards you as a friendly acquaintance.
And since they're an assassin, they got this feature:
You have advantage on attack rolls against any creature that hasn't taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.
So my player considered that, since the spell made the target friendly to them, they shouldn't expect an attack coming and, thus, should be considered surprised for the purpose of that attack only, which would grant them a critical hit (with Sneak Attack because the fighter was in melee).
I'm a bit conflicted. I allowed it for this time, but even with the advantage to the saving throw due to the combat situation, I'm afraid it could become easy to abuse, especially with an enchanter in the party. RAW, I'm pretty sure that being surprised only count for the first round of combat and that a charmed creature is still aware they are in a combat situation, even if they can't attack the charmer.
Should I continue to allow them to do this going on, or should I tell them that it was a one time fluke and eventually allow them to take another spell with Fey Touched?