How about trying to speak with the new girls parents? I had a very similar situation with my daughter. She was friends with a girl from when they first started school, but as they grew older, they had different interests. Anyway, my daughters friend became increasingly possessive and wouldn't allow my daughter to have other friends. It got to the point where I was wiping her tears away every day after school. I ended up with the other childs mother ringing me to ask why my daughter felt she needed other friends and wasn't her daughter enough?? Cheek! Anyway, after months of trying to resolve this through talks with the school and sensible discussions with the mother, I decided I was banging my head against a brick wall and moved my daughter to another school. A bit harsh I know, but it was what my daughter wanted and it was the best decision I've ever made. Several years later, she's now at secondary school, has met up with her friends from her original primary school but, thankfully, the other girl moved to a different secondary school.
If I thought, for one second, that my kids were making life difficult for someone else's child, I would be horrified and my child would be in serious trouble. I like to think most parents feel the same way so make contact - you've nothing to lose.