I am in the same dilemma...sorta. My best friend went away and bought me an ugly orange bag...it's not a fake anything...it's just some cheap bag. It was SO nice of her - she knows how much I love handbags and was being very sweet. The thing is, I only carry my LV's these days and I don't really like what she got me. I also don't want to hurt her feelings so I have made it a point to use it once in a while when I know I am going to see her. It's a tough situation. If I were you I would wear it once or twice when you know you will see her. Good luck.
This happens to me too. My friends and family know I love handbags, so naturally for birthdays or Chrismas I get handbags from them that I would not normally wear. I have a handful now that just sit there, not fake or anything, but pretty cheap and unattractive stuff (IMO). I feel guilty that I think that way and that they spent the money to make me happy. I just try to say stuff like, "Oh my, I can't possibly get another handbag because I have SO many!" So hopefully they won't buy anymore, but then they see me with another new self-chosen bag. lol Anyhow, I think in this situation, you have to preserve your friend's feelings since she was thinking of you when she got the gift. If you think you can tell her about fakes without hurting her feelings, that would be the thing to do, just so she never buys you another fake bag. (Although I doubt she would, being you don't use the first one she got.) But if you think she's the type of person to take offense, I would probably please her by using it once. (I say this but realize I probably couldn't do it myself...lol.)