@ashgate When I was younger I used to collect seed from a local pet store. I'd look at the birds for interesting mutations and let my father know if there was anything interesting and worth a look. Most of the time there were just greens and blues as mutations hadn't really taken off here at that time.
In my teens it was my turn to take a couple of birds on. I would look for interesting mutations but my heart would always be taken by the 'runt' of the cage. Dirty, scruffy and small was the order of the day. My first budgie joined me 2 weeks later, the second followed a few months later.
They both had problems, but nothing I couldn't fix.
Poor feathers? Baths and grooming made a big difference but the big changes came with a moult?
Malnutrition? A rich diet for 2 months then a proper diet sorted that.
Neither bird had any physical injuries nor diseases, and both eventually proved to be very fun and loving birds. Neither of the budgies nor I knew that I had bought them because they needed to get out of there. I bought them, but I rescued them too.
They were my first Jack and Jill, and they lived a full and contented life free of disease and other problems. I regretted nothing, not even leaving them with my mother when I left home. She cared for them the way I did.
You don't have to buy the best lookers. In situations like a pet shop the living conditions mean if there's one sick bird then all of them are either sick or disease carriers.
I say rescue this bird. Get the 2 friends back together.