It takes about three weeks or less before you get them, only if you were there before the spots crust over and just before the spots come out.
I would watch out for shingles, if you do get it go to the doctor for some medication that helps with it and get Piriton and calamine cream in just in case.
If you had chicken pox as a child that’s fine but you still have to watch it for shingles, my sister has had shingles quite a few times especially when run down. You can get it if you have a low immunity.
i wouldn’t worry as you’ve had chicken pox it’s very unlikely that you would catch it again. It is very unusual to have it twice. Both my kids has chicken pox while I was taking fingolimod and I had no problems
your ms nurse probably said stay away as your immune system is currently compromised presumably because of medication. Try not to worry about it.
Take care and I hope your grandson gets better soon.