You shouldn't get involved with your friend's ex.
Robert's friend recently broke up with his girlfriend, and Robert is finding himself attracted to her.
Robert doesn't want to make things uncomfortable with his friend.
Robert talks to his friend and asks if he would be ok with Robert asking his ex out.
Robert asks his friend's ex out and keeps it secret from his friend.
Robert's friend is hesitant, but he eventually agrees to let Robert ask his ex out without any hard feelings.
Robert's friend finds out that Robert asked out his ex and feels betrayed.