Unfortunately this is common even at 7. My granddaughter is 10 and has had friends like this since second grade. One girl is in the same grade and gets away with crying to get her way. Until this year. I went on a field trip with the summer YMCA camp and I saw the counselor turn to her, lean down to look her in the eyes and tell her to cut it out. She wiped her eyes and continued walking with him in a quiet way. I wanted to cheer! I hope her mother doesn't land hard on him. She has with previous YMCA counselors.
Because of my daughter's experience with this mother, I suggest that trying to talk with this mother again will not help and may make the situation more difficult.
The girls not allowing other girls to play with my granddaughter has gotten worse. I've learned that this goes in cycles. And that it works best to let my granddaughter handle it. I'm also a playground volunteer and watched kids to try to find a way of handling it. I discovered that this will last, at the most a week or two and then the group formation changes and someone else is on the outside.
My way of handling it is to talk with my granddaughter about what makes a good friend, how she feels when this happens, and what she can do to take care of herself and.....not do it to others. I think she is learning. Of course she's entering the tweens. That starts earlier now, too. lol She is more sensitive and cries with me about being excluded from her regular group of friends. I sympathize with her, remind her that this will change and then do something else together.
The way she seems to handle being on the ouside at school is to be tough and tell them it's not fair and run off to find someone else to play with. One time I saw this in action and one of the girls ran after her and brought her back to the group. There were 5 or so minutes when everybody was cranky but they worked it out.
When I'm around a child who is manipulating with tears, too many compliments, and/or anger is I end the play date. Immediate consequences for inappropriate behavior teaches all of the girls to be more appropriate and may have taught my granddaughter that she doesn't have to put up with it.
My daughter befriended the down and out all thru school. She came from an abusive home and felt sorry for them. She's now 30 and finally learning to make better choices. I had a difficult time with this when she was a child and for a couple or so years after she was on her own. This is part of the reason I focus more on finding ways to help my granddaughter handle this herself without judgment which I did use with my daughter and her friends. I talk with my granddaughter about friendship and what she wants from it and how she can make happen what she wants. When I'm present with the group, I coach her and her friends on a more appropriate way to act.