I wouldn’t worry about it.

It might not be him at all.

Or if it’s him, his ex might not (and likely did not) cheated on him or treated him bad and it’s just one of his stupid things. Bet he treated her like crap and lived off her and when she said “enough”, he made up all this nonsense. It’s possible he cheated on
her. Abusers and cheaters often try to shift blame. Bet he says you treated him bad too.

If you think it’s him just ignore it. Try to let it all go. Maybe get off that forum for awhile