L.,
First of all, let me start off by saying that I am not the mother of a teenager. I have a few more years to go for that. But, I have worked with teens. I'm not an expert and only have my work experience to share.
I would NOT tell my child not to tell me things that are going on. She is your only real source of information on what's happening in her life and you don't want to alienate her from you. The teen years are particularly rough, especially jr. high, and your daughter needs to know that you'll always be there for her.
As for her so-called friend, she is at an age where relationships (friendships) shift. Everyone is trying to figure out their place in the social setting of school and unfortunately some kids can go too far with that by showing off in one way or another. It seems like your daughter's friend is compensating for something and therefore is "telling others things" in order to seem more important, knowledgable, and informed than the other kids. And sadly, she focused her attention on you and your daughter. Who knows, her relationship with her own mother is strained.
You've tried talking to this girl's mother and didn't get anywhere with that. Just leave it alone for now. If your daughter continues to be harrassed by this young lady then I would limit your daughter's contact with her. And, don't be afraid to let your daughter figure out how she wants to deal with this girl. She needs to learn how to deal with those who are less than pleasant, but be there as a sounding board for her as she goes through this.
Unfortunately, we cannot protect our children from everything. Personally, I like the idea of locking my child in the house and never letting her out until she's 30! But alas, I can't do that and it wouldn't be fair to her. Still, it's tempting. I KID!!! But you get my drift right? We can't protect our kids all of the time but we can do our best to prepare them and give them the tools they need to deal with situations like this. Just keep an eye out on your child. Look for signs of distress in her life, ie. lower grades, a complete change in friends, withdrawing (more than usual for a teen), weight loss or gain, changes in sleep patterns, and such. Any of these can be a sign that she's not coping well.
I hope that this is of some help to you. Good luck and keep us posted.
K.