I would say yes if it was the police that did it. Store detectives, not being the government, would not be in violation of the 4th Amendment, although there could be state specific privacy laws that come into play.Doesn't the search and seizure of a cell phone require a warrant?
The only grounds they could remotely have possibly seized the phone over would be if they saw her texting somebody there was probable cause to believe was a confederate in the store, and giving her back the phone might have led to the evidence being eliminated. However, searching the phone numbers would have required a search warrant absent some extraordinarily exigent circumstances that one would have to spend a whole day dreaming up in connection with shoplifting.

