wel if u tell ur frend she'll assume that u are hurt and her motives have been achieved. Tell her instead that she is the best teacher cos the same thing she did to u आप are currntly returning the favour, even though its a lie. it'll sting her and she'll just apoplise and give in. thats how losers who wanna get a high ego are deflated, i mean your frend.
well at least u know how much percentage she is as freind material for the future which one दिन we all will have true फ्रेंड्स द्वारा ones side, all the way to the top. least she hasnt done anymore than talk. still be her freind just not 100 percent, good luck;-]
Tell her that आप know. If she sees you're hurt she will she's sorry and आप will be able to tell if she means it. If she doesn't feel bad she wasn't your friend in the first place.
Sit her down and calmly tell her that you've heard her/ someone told आप that she's been talking about आप behind your back. And if she starts to interrupt you, tell her, "Can आप please let me talk first?" And make sure she's willing to listen sincerely. When आप finish talking, let her be able to tell her side of the story. Listen to her like how आप want her to listen to you. Hopefully after आप both have finish telling what needs to be said, she'll apologize for talking about आप behind your back. And just to make it better, apologize yourself. Even if that may sound like आप shouldn't do so, apologize anyway. Hope this helps, but if it didn't, I'm sorry. :\