CBAvasi -
Actually, I think you did the right thing. Keeping on eye on something that you personally would not tolerate. Should you have intervened ? $1Mill question, isn't it ?
I agree with Road.not.taken [interesting moniker, BTW

] in that embarrassing someone like that would alter the idiotic mentality that it's ok to slap a woman in public. It aint. Ever.
Also agree with Sun WuKong, that unless you know how to handle personal confrontation situations, it's extremely easy for such things to spiral outta control and before you know it, YOU'RE the one taking a slapping.
At risk of a flaming [similar to the poke at Strong Eagle], I too can relate a situation where I intervened. Back in the day when I was young, fit and proud possessor of superior martial arts training, I was out running my usual route in the park. Passing a large hedge, I heard a muffled scream and it so chilled me that I actually stopped to investigate. A poor girl was being molested and the guy was about to rape her at knife point in those bushes. To cut a long story short, he panicked when he saw me, tried to slash me and ended up pretty badly "damaged" in the ensuing fracas. Police were called by some random dog-walkers [who thought I was beating up this scum], but the girl [who had emerged from her snare] relayed the story to The Boyz In Blue when they finally arrived.
I was later arrested on suspicion of ABH. The girl's testamony to the Cops save me from that fiasco. Ol' Scumbag ended up going down for nearly 9 years [after a load more rapes were uncovered].
The worst part ? Her suicide after his conviction. Oh..... and the nightmares that still occasionally haunt me.
My advice [for what it's worth] is that EVERYONE has a duty to do something and stop violence toward women [men, kids, small kittens etc]. We live in a society where such things are waaaaaaaay too common and it's unacceptable to me to ignore and walk off, for fear of embarrassment. Well done to you, CBavasi for bringing such a topic up here.