Someone at school WILL either tell her she stinks or go around talking about it behind her back. You know this as well as I do.
Could you and her mum turn this into an exciting part of growing up for her? Buy her not only a deodorant of her choice, but shower gel, more grown up underwear, the makeup her parents are prepared to let her use.
I remember how much my arm-pits and crotch smelled just before I started my periods, so check with her mum that they have talked about that - there are still girls who are scared to death when they get their first period, and others who knew about it, but are too embarrassed to mention it.
Do emphasise that smelling of perspiration is a natural part of being grown up, but even so, society expects us not to smell of sweat, desire or menstruation!