hmm. this is a tough spot.
i can assure you that this 8 year old is probably not doing it to harm your daughter. its likely that shes just copying what shes seeing in her life. so i wouldnt really say that shes being PURPOSLY abused or anything. its likely that the other kid might not exactly know or understand that it isnt appropriate behavior, especially if its something she sees, either by people she knows, or on tv. and neither of those situations can really be helped. poor child :(
what you can do is to approach the other parent in a non-judgmental way. this is the hardest part, because you are concerned for your daughter's safety, and emotional well being. you cant talk to the other mom when you are angry or emotional, so try to calm yourself and deal with any feelings your daughter has first. you can explain to her that no one is to touch her private parts (obviously, she has a clear idea of what that means!) and that she was right to come to you and tell you about it. ask HER what she thinks would be a good solution. sometimes we can be surprised by what kids come up with.
anyway, im trying to think of things you can say to the other mom. .. . you could say "my daughter came home the other day concerned with a game they girls were playing" then you can let the other mom ask you what was up, or maybe she will be dismissive and clueless LOL. either way, you could just say that there was a boyfriend/girlfriend theme and that your daughter became uncomfortable. make it about how your daughter feels, not about what her daughter did. like i said, its highly likely that the other girl was not aware that what she was doing was wrong or would make your daughter uncomfortable. so its not like she was doing it on purpose.
things you can do to prevent this from happening again is to have them hang out at your house. this way you can be monitoring what they are doing and where they are. also, you can inform your daughter that she does NOT have to go up to the other girl's room. staying in a "public" space in the house might deter any of that activity. this is not the age to leave kids alone at home either, so always make sure that your daughter is at the neighbor's house when there is supervision.
but you know, the good thing about this situation is that your daughter told you about it! :) this means that you have an awesome connection to your daughter, and in just a few short years, this will become VITAL to the safety and happiness of your preteen and later your teen! :) always let your daughter know that its great and more than ok to tell you. and you arent going to go to the other parent and expect the other girl to be punished. its probably just something the other girl has seen and thinks is ok.
anyway, hope this helps you sleep better. its tough when our kids start having adult like situations!