Sending a letter to ex?

My friend went trough that, She was in a serious relationship for 4 years and she just broke up with her boyfriend and the next they she start dating another random guy, Her boyfriend stayed friend with her, I knew he's doing that for just in case if he did get a chance to go back to her, when she broke up with her second boyfriend, he send her an email about getting back together. since then they haven't talk to each other yet. So I think Very Very bad idea, makes you look desperate.