I don't think her reaction is that unusual. Most people would keep it to themselves, but they'd still be thinking what she's verbalised. And fit and healthy or not, it's no joke caring for someone, so I can understand she's not thrilled with the idea. Is there hope of improvement?
Have the hospital given anyone training on the use of the PEG? It might be different for children, but we were told DS would not be allowed to leave hospital until a nurse had to taught us how to use the PEG and then signed off that she had watched us doing it correctly and we knew what to do.
In terms of helping your dad with toilet issues...to what extent are they expecting a 75 year old woman help a 78 year old man?
I think you need to discuss with your parents how they see life moving forward. Does you dad want to move home or would he prefer some sort of assisted living or care facility? If he wants to stay home, who will care for him (assume it won't be your mum) and how will they be paid? Who will manage every aspect of day to day life (getting him into and out of bed, showering, getting dressed, going to the toilet, eating etc), who will manage all the medical appointments, nurse visits (if you can get community nurses to assist), order the special food that goes through the PEG, who will take him to hospital for check-ups or in an emergency if the PEG is blocked or he gets an infection etc. Life's going to look significantly different for both your mum and dad from this point forward, and it's fine for both of them to deal with this in their own way, including grieving their old life. But I think you need to consider outside help.