My thougts exactly. If it's not the other lovebird, it very well could be PBFD.
If your lovebird has it, it is possible for your lovie to live a healthy and fulfilling life with PBFD. However, it's just as likely to cause a slow and painful death that deteriorates the lovebirds body over time.
I'm sorry if this does not sound great... honestly, it isn't. You need to find an avian vet and get some proper testing done. Please do not go off the advice of a pet store!