So far as I know, your niece is incorrect. Although some canonists question whether a child baptized but not raised Catholic is formally bound to observe Catholic law, that does not appear to be the case for your niece. She was baptized Catholic and received First Communion, which indicates that she was raised Catholic for at least part of her childhood. Because confirmation is delayed as late as the teenage years for Latin Catholics (Eastern Catholics receive all of the initiatory sacraments at birth), there are many Latin Catholic adults who were never confirmed but are still Catholic. If your niece is using this as an excuse to ignore her obligation to deal appropriately with her childhood faith, she should understand that it is not a valid excuse.
