M.A.
I guess it depends on how much of a freind she is to you. If you catch her in a lie, call her on it and just ask her why? Be honest with her on how you feel. If she really values you as a friend she will understand, maybe not right away but giver her time to digest what you said. She may in fact open up to you right away and all can be resolved right then. Just expalined to her that you care about her and want to help. There maybe something there she's not telling you. Good luck!