She's not his ex...
In short she assume.
Now I'm confident he is banging her. That is exactly the kind of shite a liar would say. "She's not my ex" "She just thinks we was together".
He has been banging her / probably still is / drunken fucked her a few times.
Let me guess, she is fat / Chubby or goofy as fuck. The kind of person immoral people use for a fuck when they are pissed.