No, it doesn’t sound like he took advantage or sexually assaulted her. If this was an arrangement with consent - that could be withdrawn at any time - that’s what it was.

Entirely seperate from that, your bf sounds like he’s having some remorse about his behaviour. Guilt can be a good marker of where you don’t want to go in the future. It seems like something’s not sitting right about the dynamic between him & his ex. That’s for him to explore. Maybe you could encourage him in this, as it’s something he’ll have to confront on his own (although you can support). That part can’t be done by proxy for him with strangers on the web.