In terms of actual large horses, not that many live to 25+. Lots of smaller ponies will make that age, but IME a horse that has got to its mid 20s has done well and been lucky.
A few years back, someone did a survey on horse and hound, and around 17-19 was a prime age for larger horses to be put down- those who survived this period seemed to go on into their 20s. This is all highly unscientific, but the point is that it's not actually unusual for issues to start to show up in horses in their late teens.
"Wobblers" is a colloquial term that is used about a range of neurological issues. If the neurological issues are bad enough, then vets can find signs of them through physical examination. Horses with bad neurological issues can deteriorate very quickly and there is no real treatment. They can experience sudden incredible pain. The alternative to PTS now is that he could fall and be unable to rise and potentially hurt someone in the process. With an animal as large as a horse, you also have a duty to consider the safety of those who have to handle them.
The ulcers themselves are potentially a symptom of pain or distress. If it's hindgut ulcers that are suspected, they aren't visible on a scope anyway. If the horse has already fallen over multiple times, the vet may feel it is not safe to sedate the horse and so may not be able to scope.
I get that this is distressing for you, and I'm sorry, but even if you had the money (and, being blunt, you don't, so it's very easy to say "if I had x, I would do y" without considering the actual reality), you don't sound experienced enough to care for a horse with a potentially very serious medical condition.
Personally, I think we owe it to horses to give them a good end of life. Unfortunately, that doesn't always look like years of retirement in a lush field.
I really hope you haven't communicated any of this to the horse's owner who is probably dealing with all sorts of emotions. The owner is also not obliged to disclose the whole medical history to you, especially at such a difficult time.