OP your friend is lying to you. She prolly already smashed him and is now luring you into cosigning her sleeping with a married man by making it about "revenge"....
There are a million petty revenges she/you could have come up with without slanging the kitty including printing the text messages where he begged your friend for a date and she refused and leaving them on wifey's desk so she knew the deal.... recording the married man and sending the evidence to his wife etc but no......you/she somehow jumped to doing him just because she is already suspected of doing it.... if they suspected her of robbing the bank would she go through and rob one to "get revenge too?"