Now you've said about him being in a band and having girls after him, it all looks different ... from his reaction and everything else, I think he's telling the truth.
There is one PURE fact we know here: she lied to you. She said she didn't know he had a baby and a partner, but she completely did as she wrote 'aw, cute' on a pic of your baby on FB ... she totally knew about you guys, and totally lied about it.
My theory (and bearing in mind I've just been watching 'Miss Marple' - seriously - so feeling attuned to careful observation of behaviour ; ) is:
She has a massive thing for him. She is effectively one of the FB groupies. She may well never ever have met him and just approached him on FB along with all the other girls who like him but he doesn't know.
She was out last night and she probably knew it was his birthday from social media/whatever. She may even have known where you were all going. You went home at 11.30. I don't know if you were out in a public place then went home? She could have known he was on his own by you having left (she might have even been in the same place and seen you go).
She then is by all accounts in the same club. Maybe she went up to him, danced with him, had some conversation - who knows. Probably did have some contact with him. From that, from general hanging around, she could also have known that he was pretty out of it, and known they were going for a pizza and going to his brother's - all just through being close by the group, saying stuff like 'hey where you guys going now? What you going to do?' etc.
Who knows, she might even have staggered after them to the pizza place, etc.
I think basically she knew he was on his own and she probably tried to get in the group and then got left behind. And was herself drunk, and upset perhaps. Or pissed off. Or all of it. And then ... she stayed up all night drinking and God knows what, and then at 5am decided to cause trouble and message you.
She lied about not knowing about you and your baby, which is what makes me think the whole story from her is total bollocks.
Also, it sounds like all he wanted to do was come home to you, and they stopped him. That bit of the story wouldn't be necessary if they were lying - it's not the sort of detail they'd bother to make up, I think. And if we accept that he was drunkenly saying he wanted to go round to you, I highly doubt that at that point he instead grabbed the nearest (mad cow) and took her back with him to his brother's room and did whatever. I just don't think so. It doesn't work at all with the rest of his emotions.
I think she sent the message to stir things up and to have some continued contact with him/involvement. She was obsessing over him so she decided to contact you. She probably did it on a sleepless night drunken whim, and then sort of carried on answering.
Block her, forget it - I'm sure popstars get this kind of shit every day - and I really think his reaction even sounds too laid back to be lying. And I think he'd be too hungover and guilty to hide it well.
Maybe he was passed out and she raped him. ...
She sounds a bugger. I usually think the guy is lying when people relate stories on here, but in this case, I'm voting for your guy - I think he's telling the truth.