It's not exactly clear from your question who is doing the lying, the girl or her parent. I'm guessing you're concerned about the little girl herself and the negative impact her lying may be having on your daughter. If that's correct, before you do anything, make sure you know for a fact that the girl is lying. Then decide for yourself how potentially damaging the lie really is. If, for example, the girl says "I just got a pony." Yes, that may well be untrue, but is it damaging? I don't think so. In fact, often, young children lie in order to reinforce a fantasy ("I am a girl with a beautiful magic white pony.") And/or they are feeling insecure or deprived in some way and they use lies to make themselves feel better. ("My daddy is away working on a special job and he is coming home soon with a big present for me.") Is that a damaging lie? Again, I don't think so.
But if, for example, she lies to your daughter and those lies cause your daughter to be consistently let down, or feel foolish, or disappointed, then that IS potentially damaging. And it's a negative pattern that any parent would want to know about because it's interfering with the little's ability to grow into a trusted friend.
So, my advice is for you to access the type of lie. Then talk calmly and compassionately with the parent if it feels appropriate. If your daughter is being hurt by these lies, then you need to limit the amount of time the girls spend together, until the friend learns what it means to be a trusted friend.