What happens to her when she can no longer stand or walk and her husband can't clean her or help her to the toilet etc.?

GardenA, YOU NAILED IT!
A 400 pound woman is / should be / must be / responsible for her health. She is NOT.
A 400 pound woman cannot move, clean, much less if she falls and can never get up,............only the fire department can go into the dwelling with a lift / crane type rig, so that she won't lay / lie (help with this verb, it is tough) in her WASTE... not waist.
It is so unfortunate how some people do not take care of themselves, that is a pity. And then, that responsibility gets shoved to someone else, or someone else is blamed for the 400 lbs.
Yeap, I believe this person is beyond weight watchers.............she could definitely use a gastric bypass, and learn how to care for herself. MAN, 50, she is YOUNG..................she has another fifty years to live................she could have / might.
Some kind of government entity could / might help. Not sure where, or how. Medicaid..........
Case worker
Counselor
Eaters Anonymous
................................................
The dad, living next door NEEDS help! ..........................
Too convoluted for my tired gray matter. See what others say............

M88

Perhaps the first best thing you can do for her is to find some type of weight watchers program, perhaps through a hospital, so she can address her weight, which has likely already compromised her health.

You asked: "Do you just let her lay there in her waist till she dies?" I don't know who "you" refers to, but it isn't anyone else's responsibility to care for someone who has not taken care of herself. She could still live a purposeful life if she gets help addressing the underlying weight issues.

No she was denied SSI because they said his income is over the limit for a family of 2. He has to pass away before she can collect. Dads money just does cover his bills,food etc dad can't help.

Wrong. She gets social security based on her husband's earnings. And they both are covered by his Medicare/Medicaid. And her Dad has a check too.