ANSWER:

Allah says: fear Allah to the best of your ability. 

If he does his best, Allah will accept his prayers even if he doesn’t perform wudhu or clean himself in the case he had no one to assist him.

Old person who wear diapers and can not control urine, body waste, etc. is it obliged for them to read salah in bed ridden condition. If yes then how can they read salah in un-purified state like this