Guido had DM, as did his litter mate.
I have a theory that Dm is in fact hereditary, but, esp with big dogs, we don't see it because the symptoms don't onset until about 10...and they look like normal getting old...the just happen fast...
I had pretty good lick with a raw diet and holistic therapy recommended by my homeopathic vet, though I don't remember the details.
Generally, from onset of symptoms until the failure reaches the brainstem and death occurs is 3-6 months. There is no cure. Forutnatly, since a deadening of the nerves is occurring, there isn't pain, either...
Best bet is to get as much muscle and as little fat on them as possible and walk them. Invest in a good sling, and realize that you can only do what's best for them. Not you.
Some people have tried gold bead implants, but they are fairly expensive and given the bad prognosis, not a route I considered.
I put Guido down when he could no longer do stairs and would cry all night if we slept in the bedroom, or (more the case since I slept on the couch the last couple of months) were on the computer. He also lost bladder and bowel control as the nerves died. That was in March, and I still miss him tons!
Good luck to your friend, and if you want to PM me, I'd be happy to dig out some of the info...