D.K.
I would first ask her (as a lead in?) why she thinks it's HER. After listening to her, I'd ask if she ever CONSIDERED that it might be her daughter's behavior. There are tactful ways to go about this without being accusatory and get someone to think about what's really going on.
Remind her that you DO want the girls to play together. Ask her what she'd do if the tables were turned? That might help talk out some solution.
If it were me.......I'd sit down with BOTH girls and your best friend. I'd talk to your friend ahead of time and come up w/ a plan of attack here and EACH OF YOU NEEDS TO TRY AS MUCH AS POSSIBLE NOT TO TAKE SIDES HERE. It's NOT about your or hers daughter as much as it is about the behavior. Each of you need to have a talk with your own daughter ahead of time. Kids tend to clam up when they should be speaking up. This is CRITICAL TO YOUR RELATIONSHIPS AND FUTURE PROBLEM SOLVING.
Ask each of them what they like about the other. Allow EACH EQUAL TIME to talk. Then, ask what things bother them about the other. DO NOT let the kids interrupt. Allow each to talk and then ask the other how the behavior makes them feel. You can even use other examples of the same behavior and talk about how it makes you/them feel.
What are other solutions? Set BOUNDARIES and /or goals and STICK TO THEM. Hope this helps.