As someone with a disabled partner (physically in our case) I have to agree with F3G's sentiments.
Any incident would have involved him having to assist, with no communication available at all. It sounds harsh but it's not his problem, shouldn't be his problem, and could indeed have made matters worse. That's what the relatives are there for. One of them should have been sat with their disabled family member.
I was put in the reverse of this situation. I was asked to change seats during a long haul flight to accomodate a family with a young child, which would have meant abandoning my partner to sit with complete strangers. Bearing in mind she has a neurological condition which means her mobility is significantly impaired, plus the simple act of cutting food (especially in the confined space available in an aircraft seat) is near impossible due to weakness and muscle tremors, I refused.
The cabin crew weren't happy, but tough. This was one of those very rare occasions when passenger knew best.