This age is the prime time for this. Went through it with my own daughter. And now again, at the age of 13, she's trying to figure out and deal with the various friends she has, some who don't like the others. It is a fine balancing line, one your child will need to learn and use over and over again all through her life.
No one should hone in on and believe they should be a child's "only" friend. I always tell my daughter to make friends--not just one friend. Because kids keep changing their friends and friendships so fast as they grow up, you don't want your child to have only one friend, because if that friend suddenly doesn't like her and dumps her--or dumps her for a new friend--then your daughter sits there without any friends.
My advice would be to encourage your daughter to play with both friends--even if that makes the other mad or upset while she's playing with the other one. She can even tell her friend(s) that she likes to have a lot of friends, not just one friend, and she will find time to play with both of them.
If she finds another friend or two, then she'll have more friends to play with/choose from and she won't be at the center of a tug-of-war between these two girls.
I can't really offer you any more specific advice. I pretty much told what I told you to my daughter as she grew up, and she pretty much used that as a guideline when dealing with friends. And not all "friends" are friends, per se, but some are rather more acquantainces than friends. Which your daughter will have to learn to differentiate between as well.
And all little girls say hateful things like that in their battle to make sure that you're their only friend. Is your daughter the only friend that either of these two girls have? That is also a warning sign that you'll want to make sure to help your daughter cultivate friendships with other girls, because the ones she's currently hanging out with like to monopolize their friends. They'll also think nothing of dropping your daughter like a hot potato when they find another new friend they like better or have more in common with (seen it happen).