hiding a boyfriend from your mom, is it possible?

ok guys i came across a problem a few days ago. someone i know(its not me seriously lol), got an expensive present from her BF but her mom noticed it and became suspicious. my friend got nervous and said she got it from a store(which does not sell expensive jewelry). her mom took the ring and started questioning about it to much, she got in a fight of her mom cant trust her(stupid on her part lol). now the om gave back the ring but wants to go to the store to find the same ring to see if it can be purchased there. so long stroy short she is screwed. but im trying to help her figure out a good full proof story whcih can help her get out of trouble