Oh, what a good man he is, not trying to let this woman who he didn't want, to hold his hand, sit on his lap and kiss him.
Absolutely gentlemanly behaviour to his ex, whilst disrespecting the relationship with the mother of his young child.
...and that's the positive spin!
The entirely more likely situation is that something is going on. More than you've been lead to believe.
I've been a desperate ex who wanted to get back with someone, but even then I can't imagine myself turning up on a night out my ex was having with his mates, holding his hand, kissing him, and sitting on his lap.
Even thinking of that scenario opens up more questions, where were his mates? Anyone else know a group of young men to allow an uninvited woman to come in, crash the party and start draping herself all over one of them? Certainly not the men I know.
Unless she's a very unhinged and desperate woman, this categorically did not happen remotely as he's told you.
And, it does seem unlikely they were just at the same place at the same time for this to have happened.
A co-incidence + your partner being too gentlemanly to tell someone to stop before it escalated seems all too convenient for me.