Why is my teen-age daughter stealing?...

[Brett's Honey]

Ashley turned 14 last September. I have noticed a few of my things missing from time to time, but nothing major. I have just taken my things back from her bedroom. I should have talked to her about it then, but I didn't. Yesterday, a good friend of mine came by to tell me about last weekend. Ashley was spending the weekend at her house with her 15 yr old daughter. Before we picked Ashley up, my friend and her daughter had gone through Ashley's bag and retreived several things that she was trying to steal - 3 big bottles of body wash, new hair brush, lots of new make-up, a new comb, several CD's, and even some clothes. My friend's daughter has also has heard that my step-daughter and another friend of hers have been shop-lifting. Apparently her Dad didn't let her go to the sand drags today, I haven't had a chance to discuss this with either of them yet. I can't beleive she's stealing from her best friend! (She is normally a very nice, helpful, good kid.She has a ton of friends! A little lazy, but that's all...)
Has anybody here ever had to deal with a teen that steals? I know that 14 yr old girls have a whole lot going on with life in general, but this stealing has got to stop! She's on her way to losing friends and probably getting caught shoplifting, too.
Any advice? Please...............and thanks!!