I had that in 1981. I got up one day and my legs didn't work properly, so by the end of the day I was in hospital. After a few days I'd completely lost the use of my legs. By the second week I was taken to another hospital and put into intensive care as I was getting close to the stage where I would have needed to go on a respirator.
Luckily for me, this didn't happen.
As well as losing the use of my legs, I had only partial movement of my arms, I lost the fine sense of touch in my fingers (which made it hard to read as I couldn't turn the pages properly), I couldn't swallow properly so I had to be fed through a naso-gastric tube. The left side of my face was heavily weakened, and I could only really focus on things in front of me.
I spent two months in hospital, towards the end of which time I had to learn how to walk again (which with muscle wastage and tendon shrinkage Really Hurt!) and graduate from a walking frame, to crutches. Then after two months I was sent home, and had an ambulance take me to physiotherapy a few times a week, until I graduated to a walking stick and was able to go on a bus.
It was about nine months after I came down with it that I was able to walk without a stick.
It's not pleasant, in fact I'd have to say it was the worst thing that ever happened to me. But I made a full recovery.
Best wishes to your relative, A2QFI.
As Alamo says, be optimistic!