My friend is on Federal felony probation. She was recently arrested for a misdemeanor shoplifting. She did not shoplift anything. As a matter of fact, she had made purchases before leaving the store. What will happen to her if she is convicted on this bogus charge with her felony probation?

I think your friend isn't being completely honest with you...(and only a fool would tell anyone the truth before trial) they don't issue arrest warrants unless there is some evidence to support it. When it goes to court the determination will be made if that evidence is enough or not to support a conviction. The DA doesn't waste time with cases that have no evidence. So obviously they have evidence (meaning they got her with it) that she was either concealing merchandise to remove or has actually taken that merchandise out of the store and didn't have a receipt to back up a purchase. They might also have security camera footage or theft prevention personnel eye witness testimony.

One ploy some shoplifters try if actually buying an item....then coming back a little later with the bag and receipt and steal another, claiming it was bought earlier.....many stores have far more video surveillance than you might think.

Let her and her lawyer deal with this.

Then why was she arrested?

You are naïve and she is lying. I don't see what any purchases she made have to do with the shoplifting - proves nothing. If she thinks she's going to have more than about one minute to defend herself in court, she's dreaming. Shoplifting cases are slam dunks.
Her first act should be to tell her PO about this. Hiding it will just make it worse. We can't predict what the PO will do, or what the judge will do.

Then why was she arrested?As joypulv states... your friend is flat out lying to you. Stores don't go after people unless they actually caught them in the act with the items. In which case... as was said... its "a slam dunk". They don't have time or money to waste on suspicions.....they do it when they have someone dead to rights.