Mystery infection

A work mate of mine has a young hen (probably around 10 months old) who for the last month has had issues with her nose being swollen and pussy (apparently at one point it was so swollen that she couldn't see properly). Vet couldn't come up with any distinct cause - the hen was on antibiotics for a month but they made no difference, she just continued to deteriorate.

It has been reaching the point where it is kinder to put her down than let her suffer, and today they woke up to find that overnight one of her eyes had turned into a swollen ball of puss (it was quite gruesome)! So today my work mate brought her to work (we work at a vet) and had her put down.

I'm just wondering if anyone has any ideas on what her issue could have been? The sick hen was not in quarantine while sick and none of my work mates other chickens have contracted it. We had a look in her mouth after she was put down and saw a gap in the roof of her mouth and wondered if she had a cleft palate but then the vet informed us that all chickens have a gap in the roof of their mouth... This particular hen's one looked a bit manky though, it had food and other gunk stuck in there...
Any ideas on what was going on with her?

If there were mosquitoes out, then it possibly could have been fowl pox. Fowl pox can cause scabs around the face, comb, wattles commonly, and wet pox can cause yellow lesions inside the mouth and throat, even down into the trachea and esophagus. Her roof of the mouth has a normal cleft. Here is a good link that shows this along with a pic of the roof of the mouth: http://www.thepoultrysite.com/publications/6/diseases-of-poultry/195/fowl-pox/