I want to tell my ex's new boyfriend she's cheated on him
She's your ex now. Why are you involved in her life?
I want the new boyfriend to be unhappy like I was.
If that's the case, then go for it.  People deserve to know when they're being cheated on.  I hope your ex cleans up her act!
Well she did cheat on me, but this isn't about that.  I just want to watch him suffer.
It is understandable to want to warn somebody so they don't get hurt. You should be careful not to do it on purpose to hurt him. You would not like being hurt like that just cause someone didn't like you.