The girl's mom can't do anything about this. You might talk to your daughter.
"Susan, it's too bad Marie is acting this way. I know people who are like that, too. She doesn't realize how much she is hurting you. She doesn't understand. You see, for a long time Marie has been your buddy, and what you may not realize is that she didn't pay much attention to other nice kids. Now she has a couple of other friends and doesn't have time for you. It hurts, but it is nothing YOU did. You just know something about Marie now that you didn't know before.
"Do you know about Mr. Toad in "The Wind in the Willows"? You can read the whole book when you're older, but Toad is a very nice, friendly fellow who simply can't be interested in more than one thing at a time. When he likes boats, he's crazy about boats - thinks about boats, talks about boats, and doesn't like anything else. When he likes cars, he won't have anything to do with boats.
"Marie is being a little bit like that.
"Would you let Marie go away and do her own thing for a while? I'm very glad YOU can be friends with many people at the same time. This is your chance, if you'll take it, to know those other kids better, and they may need a friend like you. If you'd like to invite some of them over, I'll be glad to call their moms. This might turn out to be something good, no matter how sad it seems right now."