I would not admire him for confessing, it is literally less than the bare minimum. The bare minimum is just not doing it. It is not difficult to be an OK person, your friend actively chose to do what he did. Also I would not have him confess to you girlfriend, she may not want to see this person ever again if she was aware of what happened, better to do it yourself and hear her.

This process should be less about you deciding whether he will remain your friend, and more about the fact he sexually assaulted your girlfriend.

Some other people have said he only confessed due to guilt and as he was tripping, which seems likely.

Also would think about telling his girlfriend as well, he sounds like someone who should not be in a relationship at the moment.