It's bad to date someone a friend has dated.
John is single and knows his best friend Matt just broke up with his girlfriend Mona, who John has always been attracted to, and John and Mona still talk.
John wants to get in a relationship.
John decides to ask Mona if she has any single friends she could recommend him to.
John decides to ask Mona out on a date which goes well, and the two start seeing each other regularly.
Mona sets John up with a friend of hers and the two start dating.
Matt stops talking to John after he finds out John is dating his ex.