It really sounds like you all are handling this really well given it just got sprung on you.
Obviously she should not be alone with this other child.
Speaking as an abuse survivor (and not an expert)...I don't think she will be 'scarred for life.' It may be that she will continue to process it and as some time goes by you'll hear that it really upset her, and at that time you might want to consider therapy (if it happens).
More likely, she'll remember that she was involved in something that was disturbing, and she was able to talk to her mom about it and you guys totally supported her in her feelings that it was not a nice game, without making her feel guilty, and she'll just get on with things. One of the worst things about abuse, particularly as a child, is not being able to express what is going on or have people listen. In this case your daughter has that. I think it will be okay, as long as you are willing to listen and watch for signs that she might need more support.
I would maybe bring it up once or twice more but really gently. Just maybe when an opportunity arises, like a bath, to talk a bit about how it feels when someone touches us somewhere we don't like, like Friend did. I wouldn't probe or push, just let her know it's an okay topic.
If she has nightmares, shuts down, stops enjoying normal activities, or can't seem to stop talking about it, those would be signs to look for more support I think. Probably someone has even better info on that.
I would personally not tie it to this event, but in a few weeks I would roleplay with her around saying no and how to handle friends who say things like that. Friends who say you have to do something to be their friend are not good friends. What words you can use, and so on.
~ Mum to Emily, March 12-16 2004, Noah, born Aug 2005, Liam, born January 2011, and wife to Carl since 1994. ~