my girl and I are both competitive powerlifters. We only buy inzer belts and she loves them. She uses a 10mm lever belt. I would recommend 10mm for a female, should be more then thick enough, and the lever belts just make life so easy. Hers was around 100 bucks I think from inzer's website.
In my opinion, when it comes to powerlifting belts, inzer is simply the best. it can take a month for a standard black belt to come in, and up to 6 months for colors though.
don't get her a generic brand.