I'm going to tell my friends that my ex flat mate cheated on his ex gf.
Are you jealous of your ex flat mate?
Yes I am and the cheating really hurt the girl.
Even if your ex flat mate was cheating, it is not your place to bring up gossip about him. Don't talk about others when they are not around.
I'm going to tell my friends.
None of that is your friends' business, you'll just be spreading gossip by telling them. Don't gossip about people it's not nice.