A vet check should include a quick eye check but would not normally include scoping for ulcers. However, a vet may not be willing to give you information about some-one else's horse.
The problem is, if something is found, are you willing to pay for potentially ongoing treatment for this issue? If he has ulcers, for example, he could end up needing costly medication long term. If he has kissing spines, there are various treatments, none of which are cheap. Most problems, at the very least, would need time off work to allow him to recover.
If she has sharers, is she relying on the financial contribution from them? If it turns out he has a condition where he needs some time off of work, then the other sharer may walk away, leaving her out of pocket.
Also, if she wants to sell the horse, and finds out that he has a problem e.g. poor vision, then it ends up becoming a complicated situation, as you've arguably reduced the value of the horse by finding this out (yes, the new owners might have got him vetted, but not everyone does this, especially if the horse is being advertised cheaply).
Unless you are willing to take the horse off her hands for a nominal fee and do the veterinary investigations yourself, then I think you should walk away from the situation and say why. It may give her a bit of a wake up call.
I can understand not wanting to do this and feeling sorry for the horse, but honestly, unless you are willing to potentially fund vet treatment long term, finding out the cause of the problem could just make things worse.