I think it would be ok, but I'd probably steer her in the direction of martial arts or kick boxing.

My main reasoning is tha whenever you see woman kick boxers on telly they are all lean and sexy as hell, yet whenever you see a woman boxer they look like men with a slight waxing

The best sport of all though for a girl I'd imagine would be to combine swimming, martial arts and gymnastics so that she could go into Miss Fitness competitions etc.

You want to steer her in a feminine direction, boxing is a bit of a butch sport for women imo.