A parenting question for all of you

We just discovered that one of my daughter’s “best” friends has been lying about her computer use to her parents. She tells them that she’s going to an approved kid website, such as Club Penguin, and then, when they’re not looking, goes surfing for sex sites. (Did I mention that she’s ten?)  My daughter, bless her heart, told us what’s going on, because she was made very uncomfortable when shown a website with “naked people.” My daughter has no problem with our decision to limit her contact with that friend so that their out-of-school interactions (which my daughter hopes to see decrease in frequency) happen only at our house.  So that’s not the issue.

Here’s the issue:  I think I should tell the girl’s parents.  My daughter is, of course, worried that her friend will figure out that she is the source — even if the parents claim to have discovered the problem through browsing the computer’s history.  I appreciate my daughter’s concern, and I certainly don’t want to turn her into a social pariah, but I still find unnerving the thought of a child, too clever by half, wandering around alone in the big, ugly world of the internet.  Do you think I’m right to want to tell the parents?  And if you do think I’m right, how would you broach the subject?  Should I recommend to the parents a strategy (such as saying that they discovered the problem by checking the computer’s history), so that my daughter doesn’t get nailed as the stool pigeon?  Or do I just reveal the problem and hope that the parents don’t disclose my daughter’s identity?  I’m worried that, if I just ask them to keep my daughter’s identity secret, without giving them a strategy, they’ll do something stupid such as saying, “One of your friends told us that you’re checking out bad places on the computer.”  The other girl would have to be a complete nincompoop not to figure out that the friend is my little bookworm.