Wizard & Charm

[Fufu]

Good question! A wizard not involved in Combat cannot "Charm" another player's Monster.
Munchkin FAQ states...

Q. Can a Wizard use his charm ability if he's not involved in the combat?
A. No. The card says this is something he may do instead of fighting a monster. It's not for interfering with others' fights, so the Wizard in question must be the munchkin who opened the door or the helper.