I'm just finishing up a PhD in philosophy, so maybe I can apply some 'critical thinking' to this issue of child brides. As an aside, what is up with this guy pontificating about philosophy, sociology etc., when he did a bloody medical degree? Does he think that because he's clever enough to get into medical school he automatically gains knowledge of these other disciplines? Anyway, if he'd read any philosophy, he would learn that almost every single philosopher agrees that cultural relativism, as a theory of ethics, is bullshit. They've agreed this since the late 1960s/70s. And philosophers hardly agree on anything. So the fact that 99% of professional philosophers think that cultural relativism is bullshit should give you some indication of how illogical and nonsensical a theory it is.
He'd also learn that the vast majority of philosophers agree that ethics is objective - that some things just are right, and some things just are wrong. Very few philosophers think that ethics are just people's personal opinions, man. And so very few philosophers would agree that FGM, or marrying off child brides, are bad just because, like, colonists, like, have the opinion its bad. Rather, he'd learn that most philosophers think that marrying off child brides, and FGM, are bad because these are objectively morally wrong actions. That isn't anyone's opinion. That isn't just a matter of taste. These are really, actually, objectively bad things to do: period. These actions aren't morally permissible anywhere: they're wrong in the UK, and they're wrong in Sudan.
One final point. When he talks about culture, whose culture is he talking about? The little girls'? Because I'm pretty sure the little girls don't get together and decide what their culture is, and all nod and agree that, yes they'd actually quite like to be married off at 12. It's not the girls' culture because they don't get any say in this. I'd bet that if we could ask those little girls they really would rather they weren't getting their genitals cut, or being married off to 50 year olds.