Was he ever intimate with her?
How old are all of you?
Was she with someone else that might have prevented her from doing so? Maybe he mentioned it to her, or she noticed something from you when she did it? Either way, it's highly inappropriate, and you are not overreacting at all. I would explain to him that she is not his woman, and he has no idea where her lips/mouth have been, and its unfair to expect you to kiss him when she has put her mouth, where only yours should go. Don't accept that jealousy excuse.
I welcome your thoughts...let me know if you want to talk more