Quote -- "my question is, how much trouble could she be in for what was an honest mistake?" "She was totally thinking at this point that she had returned the merchandise and everything was fine." Bullhockey! This is a dreadful story. She needs to skip it and tell the exact truth without the excuses like "I was tired" "I was in a hurry" "I had several items to return." They just make the story seem worse.
A chat with an attorney is definitely in the cards if she is further contacted by the store. They will tell her not to use any of these feeble excuses.