It's bad to fall in love with your friend's partner.
Tony has hung out with his friend and his friend girlfriend a lot recently, and has begun to be attracted to her.
Tony wants to fall in love with someone.
Tony decides to try going out more on his own and get to know other girls.
Tony continues to spend time with her and falls in love and confesses his feelings.
Tony find another girl and eventually falls in love with her.
his friend and his friends girlfriend both decides to distance themselves from Tony and not be his friend any more.