The girl was an invited guest, yet in your home. How do we treat invited guests and how do we act in other peoples homes. I was taught, we overlook minor mistakes that people make, because they will not be there forever anyway. On the other hand, I was taught we use our manners at all times and we don't misbehave.
Well, they are kids, perhaps they were up too late, and perhaps loaded with sugar, just a thought.
At 11, I think they are still learning right from wrong and understanding how to react to certain people. So what you may get away with in one home, may not reflect what is allowed in another home. Your daughter knows the rules, however, on the other hand, she is 11 and cannot control what everyone does. Keeping in mind it was a birthday party.
I saw your post, but do not remember the date of the party. In any event, if it was Saturday night and today is Tuesday, it is time to let it go. They are kids.
I don't blame you for ignoring the girl. It is not your place to teach her how to act while away from home, that's her parents job. And if she can't listen to an adult, then there is no point in answering her calls, texts, or any other plea. I don't really know what else you can do, but ignore her. After all, what would we do to Jimmy when he wants to play with Johnny, the two 5 years olds who recently crushed each others Tonka trucks? We would tell him Johnny can't play right now. I see it as not much difference.
As for your question, to confront her or give her the silent treatment, I think you already did confront her. Perhaps you could take one of her calls and tell her, you will have Jennifer give her a call when she has a moment. In time, perhaps she can do that. If the subject comes up, Jennifer can tell Lisa she just doesn't like gossip, but it is over. If that doesn't work, Lisa is not a good fit for Jennifer. If Jennifer can use this as a future guide to picking only the best of people as friends, she has figured it out early.
As for the stress from the AWFUL BIRTHDAY PARTY, it is time to let it go. If that is one of your biggest problems in life, consider yourself lucky.
No offense intended to all of the Jimmy's, Johnny's, Jennifer's, or Lisa's in the world.