First off, ouch. Second off, that is very shallow. But not going to lie, I've been there. The best thing to do is to be nice about it and let her down gently. There is no way to avoid hurting her because lets face it, nobody likes to be told that they're unattractive. The least you could do is add positives to breaking up. Don't cut her off right away, but I'd say at least be there. From my experiences, whenever I'd say we could still be friends (and I did try to) everything ended a lot less messy and hurtful. Best of luck.