Yes, a double edged sword is probably the perfect way to describe it.
And in those three years, dfil did have some lovely times; there was, to a degree, still some pleasure in life, and it better for him than the alternative.
I think you're right to support her if that's the route your mum wants to go down.
Are you going to the renal appointment with her? If I may make a suggestion, if it goes ahead, I would try and avoid hospital based units. Towards the end my dfil attended a unit that was further away (but patient transport provided) but was dedicated to people on long-term dialysis. This ran much more smoothly than the hospital one as they didn't have the pressures of accute patients and could actually run to a schedule.
It also meant that as it would be the same people on at the same time, he got to know them a little bit and could chat to them, which made the time pass quicker.
I don't know if overnight will be considered - he was offered the option of five nights overnight dialysis on a machine at home. Initially, this sounded the better option to us, but once he started having dialysis at the hospital he decided that because he felt so bad after, it was better to have four dialysis free days than two, and he doubted he'd sleep when plugged in.