What are you approaching? Her kissing someone or her kissing a girl? If it's the latter, don't. Let her come to you.

I’d leave it alone unless it escalates. If it went further maybe but I wouldn’t come out with the fact that she was caught on camera doing that. It could be harmless. Try to stay open and positive. There are worse things she could be doing at 15 😊

Oh wow approach it gently I guess. Im pretty straight with my step daughter if it's like that I go where she's comfortable and come out with it. Just tell her you seen it and see if she explains. When she does wrong I confront her not with anger either.