Exactly... Lack of evidence, which is not worth reporting!
Why should she worry about lack of evidence when she has been violated in one of the worse possible ways, by someone she trusted enough to fall asleep in his home? How many other times has she fallen asleep & had him abuse her? Does he abuse his wife in the same way? Any more of his wives friends he has violated & who haven't reported it due to lack of evidence?
This guy assaulted her while she slept, with his wife just feet away. I would be VERY surprised if this was a one off! You need balls to be confident enough that you will 1] get away with it & 2] not be caught.
It would really surprise me if this was his first time doing this too, not necessarily to the op, but to his wives friends.
Crimes with lack of evidence are worth reporting.
What if this guy has form & from op's description of the attack, they can identify a serial abuser? Long shot, I know.
There might be lack of evidence if he hit his wife, but she could still report it so the police had a record of his attacks if ever they were needed. Again, nothing to suggest he beats his wife, but it is possible.
Not only that, but the op has been assaulted. If she didn't know this guy & it was a random someone who did it while she was on public transport, would you say then same thing?
I think op should report it because she doesn't know if she is the first, she probably won't be the last.