I think that dollytwat is right, and you should speak to the teacher about this. Firstly it could really blow up in your face if you tackle your friend directly - how will you react if she disbelieves you? If you talk to the school, they can keep an eye out for nasty behaviour, and nip it in the bud there and then. Secondly, as the behaviour is happening at school, that's where it should be tackled.
I had an issue like this with ds2. When he went to senior school, there were only 4 other boys from his primary school who went up with him, one of which was his best friend - they've known eachother since they were 18 months old, and his mum was my closest friend. By the time the boys were in year 8, this other boy was joining in with bullying ds2 - verbal bullying mainly, but having already been bullied at junior school, it really upset ds2 - especially as it was his supposed best friend.
This lad had also basically kept ds2 as his 'spare' friend - he made plenty of other friendships in the school but never thought to invite ds2 to hang out with them. Ds2 was for when no-one else was available (I didn't see this until it was too late). This meant that ds2 had no other friends at school, and was totally isolated when this lad started bullying him.
I did tell my friend what her son was saying to ds2, and she flatly denied it. She said her son had denied saying anything nasty, and he wouldn't lie - the clear implication being that ds2 would.
Thankfully, within the year, we had moved to scotland, where ds2 has a big group of really good friends, and is thoroughly happy. We have seen the other boy since then, and absence has made things better, but I think the whole thing damaged the relationship between me and my friend.