The correct answer is, of course, you should never grope someone, no matter what they are wearing or how they are acting.
There is no excuse for that kind of behaviour. "She was topless" is not an excuse, or permission.
I read the comments online from people saying "Well, she was asking for it going topless around a crowd of young boys" and I wonder - would the person who said this have grabbed her breasts?
I can almost guarantee you these people would be offended by having this question put to them. "Me? No, I'm not that kind of person. But others ..."
That in itself should tell you these commenters, on some level, recognise this kind of behaviour is wrong.
It reminds me of the Egyptian lawyer who was jailed last year for saying women in ripped jeans should be raped.
On television, he said: "I say that when a girl walks about like that, it is a patriotic duty to sexually harass her and a national duty to rape her."
You would think people in a more progressive society would eschew that kind of thinking.
But the comments surrounding this story show we're not as progressive as we may think.
The woman is still waiting for her apology. She deserves one.