My daughter Peanut is about Babybartfast's age and is in preschool. She had a problem with one of the kids in her class playing too roughly, and her teacher finally told her that it's okay to refuse to play with him. She told her that if they're playing and he's rough, she can just walk away. And if she just doesn't want to play with him in general, she can walk away.
She's responded really well to being told she can control her interactions with the rough kid. And he's picked up pretty quickly that she won't play with him if he isn't gentler with her. It's actually been better for both of them.
I guess that's my long-winded way of saying that you should follow Babybartfast's lead. If she wants to end the playdate, then there's no need to protect the other mom from the reasons why. You can do it without sounding judgmental -- "the kids play more roughly than what Babybartfast is used to" is pretty straightforward without sounding like you're judging their level of permissiveness.
Also, if you don't want to leave just yet, can you just bring an activity that she can do by herself? That way your friend's kids can keep playing, you can keep visiting with your friend, and Babybartfast can entertain herself.