I don't think you have to do anything but listen to your friend. People get to make their own mistakes. It doesn't matter what the actual deal is that is going on. It is not your problem. Being horrified or not is not, to me, the issue here. Be horrified if you want or tell your friend how you would handle such a situation if it happened to you -but beyond that - I don't see what it has to do with you or your reaction. It is your friend's problem. I don't mean that coldly either - I mean it as don't get meshed up into something that is not yours to get meshed up into. Support your friend by listening and other than that, stay out of it.