We (1by1) tend to medicate for minor problems immediately upon admittance of the cat into our group. (ear mites, fleas, worms). This reduces episodes like this one.
If the cat is found to have an intermediate problem, that went unnoticed, like maybe a few bad teeth, we do offer to pay for vet help. (our thinking is that if the cat were still with us, we WOULD be paying....right? So why not keep it in its new home?)
We have has similar incidents with LARGE problems too, even if they could not have been anticipated. Same thinking, if the cat was still with us, we would treat it.

Our adoption agreement states that we do NOT guarentee health or temperament! But when these things happen right after the adoption, we feel obligated to help (like we should not have missed the problem in the first place). I believe this helps us to have a good reputation among the population.

Other organizations in my area hold fast to that NO guarrentee thing. You are on your own!!!