Firstly, cassa333 appears to be a mind reader by knowing what was in the security guard's mind! That's a very rare talent!
Secondly, geaytea, let me tell you quite categorically that, according to the details you've given, your friend has NOT committed the offence of shoplifting. Sounds to me as if she may have "panicked" on seeing the security guard and she therefore decided to put the item back. She, therefore, simply DID NOT commit the criminal offence of shoplifting.
The store staff still had NO AUTHORITY to question or interrogate her in law. After all, had she made any admissions to them, and the Police HAD been notified, the store would undoubtedly have used her admissions to them as part of their evidence. The store did however have the right to ask her to leave, as they could anyone for any number of reasons.
A Police Officer would realise that no offence had been committed. The Police could have given your friend a verbal warning about her future conduct but the matter would have ended there.
And don't forget, the store's version may differ radically with your friend's account of what happened. I do not know the complete facts, neither do you by the sound of it, only your friend and the store staff do.
I'm concerned that the store are apparently sending a copy of the banning order to her home address because I can only repeat that, IF you version of events is accurate, then the store had no right to demand ANY details from her, let alone threaten to send a letter to her home. I could only suggest that she doesn't have to divulge details of such a letter to her uncle or anyone else if she doesn't want to, difficult though she may find that to do.
If her uncle is as unapproachable as you say, then all your friend can do on this occasion is to put this down to experience and