Response to OP: (pardon is this has been covered, didn't read all of the posts yet) Address hypothetical as husband/wife/other man friend.
1. Stop her before she even got that far. Letting it go that that far places part of the guilt on the man also. Begs the question "what kind of friend are you?".
2. Tell her that if this happens again you will tell the husband, whether it's with you or you hear about another.
3. Don't tell the friend but talk to him about the husband/wife relationship he's in. Hell, for all you know he thinks it's OK.
1. Stop her before she even got that far. Letting it go that that far places part of the guilt on the man also. Begs the question "what kind of friend are you?".
2. Tell her that if this happens again you will tell the husband, whether it's with you or you hear about another.
3. Don't tell the friend but talk to him about the husband/wife relationship he's in. Hell, for all you know he thinks it's OK.
Being told you're delusional does not necessarily mean you're mental.