If they are not prepared to pay any cost for vet treatment they shouldn't be allowed them! If you take a pet on and it needs something done it shouldn't matter what it costs! Before stressing the creature out they could've at least phoned up for a quote!
If he couldn't eat for a while he should have been seen to before now and leaving him to starve before anybody realised or thought to do anything is inexcusable!
It's not a conincidence that he died just then, as other people have said guinea pigs get stressed VERY easily, as he was old, stress should've been kept to a minimum and since he'd not been able to eat he would've been feeling low anyway. It didn't kill him outright but it did push him to the limit.
He was eating, grass, straw and his food in the bowl, just no the vegetables. I think my parents are prepared to pay whatever it takes, that is evident from when they took the rabbit to the vets.
Also I know diarrhea leads to death but no one would have knew as there was one path that was no there the night before, I noticed it when I seen him in the morning laying there, dead.