These people land on neither side of what I proposed, except that their taxes are subsidizing Person B, causing them to have less money leftover for things like rent.
A 25 year old with medical problems that is able to work full time is good news; the fiscal penalty of the reality is likely just their yearly deductible and time lost to medical appointments. Thankfully they can work!
Losing 2/3 of their income to 1/3 of a rent share makes it seem like they should move, however. Our that they should try to find a better paying job.
Is ypur point that this person is in such a destitute situation that they cannot possibly save for retirement given 40+ years of career ahead of them? Does this person have no family upon which they can lean? Do they not belong to any community that can help? Do we expect someone with their medical condition to even make it to their golden years?
This situation seems unlikely for the majority of folk. And I always recommend LTD insurance to everyone, regardless of whether it would apply here.