For some time now, I've been toying with a vague idea of handling Charm Person/Monster spells cast on a PC using a method similar to the one described in I Meant to Do That and GM-Ordained Betrayal. A vampire's victim can be handled using the "I Meant to Do That" approach, but most other Charm situations aren't mind-control; they turn the victim into a loyal friend (highest possible reaction roll result.) You don't roll for PC reactions, which makes Charm Person cast on a PC difficult to handle.
Well, we could just tell the player "you've been Charmed and now consider the creature your friend". Most players will go along with that. I wouldn't bother keeping it a secret, either. But also, I would make it clear that the player can do anything they want, but will be saddened or feel despair if any harm comes to their friend.
I'd base the sorrow mechanic on the system I suggested for aimed blows. Any damage done to the charmer has a chance (damage result 5+) of causing emotional pain to the charmed. If the charmed PC caused the damage, the PC is now "miserable"; if someone else causes the damage, the PC is "angry". If the harm does not involve a damage roll, roll a d6 anyways. Modify either result by the charmer's Charisma modifier.
A miserable or angry PC is at -1 to all actions until the PC takes steps to make amends or avenge the "friend". A very miserable or very angry PC is at -1 until either the target of their misery or rage is gone, a friend successfully talks them out of it (5+ on d6 roll,) or the spell is broken. If repeated emotional trauma moves them beyond "very miserable/very angry", the PC is wracked with guilt and despair; treat this as a wasting disease (1 hp damage per day until the PC dies or snaps out of their bleak state; roll 1d6 daily, modified by Wisdom, to see if the victim recovers.)
The player is informed of the character's changing mental state, but the GM never takes control of the PC or tells the player what to do. The player can do whatever they wish, even attack the charmer. If the player is creative, the penalties can be sidestepped or overcome.