I think the real key there is actually in the "the other day he told me" part.
Was this a feeling so guilty he had to offload it, or asking you for advice on what to do, or was it basicly bragging about it?
Given the huge lack of detail and the human tendency to imagine the worst possible case I think the way he was telling you this is actually very important for figuring that out.
Also, he is still her friend too, tell him to admit it to her. Part of something like that being traumatising is how much time she has for her imagination to come up with possibilities which contintue to develop the longer they remain uncertain. If she can ask him for specific details right away its less psychologicaly distressing for her.
Actually, a good time for him to admit that is next time they get drunk together, it takes the awkwardness out of anything and she'd tell him what she really thinks.
Lastly, if she was so drunk she passed out, chances are he was almost drunk enough to pass out.
I've known people who thought it would be a good idea to bite through their glass or try to jump off a roof when they were that drunk. (niether needed an ambulence luckily)
Alcahol does have a habit of messing with the "that is a totally stupid freaking idea" filter in your head. This isn't something he did sober.
