If it were my daughter I would tell her that her friend seems to have moved on and no longer wants to be friends, for whatever reason, but doesn't know how to tell her. And, I would ask her if this is the type of person she really wants to be friends with. Your daughter can't do anything about her friend's behavior, only hers, and walking away from the friend and making new ones seems as if it would be best for all, even if they are on the same team. She can still be polite to her, but leave it at that.
It's a sad lesson to learn, but it happens. My nephew, 11 this July, was good friends with a boy for a couple of years, they did a lot together with each other's family and at school, and then about a year ago the friend began to make snide comments about him at school, stole some things while here at our home, and lied about it all to his parents. Talking to them did no good whatsoever, they actually stopped answering my sister's calls, she was still willing to have him over because she knew the family dynamics...dad out of work, mom overworked, money tight, attention going to the two older siblings in high school. My nephew was the one to say he no longer wanted to be friends with this boy anymore and moved on. This year in 5th grade they're in the same class and the boy has tried to stir things up, telling my nephew things and being mean to his friends, yet acts like they're still friends in front of his parents, so he knows what he's doing. But my nephew has grown from the experience, knows that friends don't intentionally hurt you, and is polite to him but keeps his distance.
I know it hurts to see your daughter hurt, but talking to the mom probably won't do a thing other than increase hostilities, even if they're more passive aggressive ones like the friend is showing. Reassure your daughter she's done nothing wrong, tell her that people change, and friends change, and it's normal and OK. What's not OK is to allow yourself to be treated wrongly. Use this as an important lesson in helping her build her self-esteem.