i think the treatment is going the right way. they really do need the dental xray - there is a possibility that his dental roots have overgrown through the top arcades into his ocular spaces. this would not be good so fingers crossed it's something else. it could also be an infected root given he has a lesion they can see but again, upper jaw is not a good place to have that. the ulcer they can see could also be caused by a spike/spur on his teeth - this happens when the teeth are not ground down evenly and as it's sharp, it can cause injuries on the inside of the mouth. they would have seen these, depending on how good he is at letting people have a look in his mouth. these can be filed down and if his teeth are not misaligned, tooth friendly diet can help keep these down.
as for fresh fruit, i am not sure it's location dependent. i think there are plenty of people who don't know what healthy treats are (fruit sounds healthy!) and because there quite often is no immediate obvious problem, they think everything is fine. i've heard of people who gave their chins chocolate as a treat without any harm. all you can do is give them the information on what's healthy and what isn't, there's sadly not much more you can do and if people decide to feed their chinchillas nuts, fresh fruit or, in case of my rescue banana chips and no hay then there's a good chance they are making them sick.
if nothing else, eating high calorie foods like fruit mean they will eat less of the low calorie foods (hay) that they desperately need to keep their teeth down.
just to illustrate, mae, i rescued her last october and she had about a week left in her. she was so undernourished she could not even have a dental xray she desperately needed. the people who owned her before fed her banana chips and chinchilla mix. she had numerous spurs, a fragmented tooth, she has a chronic jaw bone infection which has caused the jaw bone to extend downwards (she has a tusk), she's been in gi stasis numerous times and she's frequently getting respiratory infections. ALL this could have been prevented if she was being fed the right diet. she's not suffering from dental problems anymore since her dental and i have her on a special dental diet (we have to be very careful what we give her because of the constant stasis danger). she's a happy and affectionate girl and thankfully i'm pretty good at recognising there's something wrong so we start treatment straight away - this usually means that she only feels rubbish for a day.