I'm going to tell somebody that their girlfriend is cheating on them
When will you tell him that?
Whenever I see him next.
Letting someone know they are being cheated on is the right thing to do. Being unfaithful shows they are not committed to the relationship.
She will hate me and I don't care. I just want her to get dumped.
If she is really cheating then her relationship should end. If she were the one being cheated on she would want someone to tell her.