You're right, it could have/should have stopped at the first rebuff. But it does make quite a lot of sense that someone who we are already assuming is very bad at reading attraction and social clues might miss or chose to ignore the first rebuff, especially when it is an indirect rebuff like the one in this story.
"I try giving him the benefit of the doubt and make some quip about his being a friendly sort"
To be fair, we don't know exactly what she said, but I can imagine a few ways that she could have said this that would have come across as playful, not corrective. I'm not accusing her of anything of course, but combined with the fact that you pointed out that this guy might not have the social intelligence to pick up on the hints, anything that could be construed as playful would have been.
It's still his fault for missing the hints, but it definitely turns him into socially inept, not sexually assaulting.
