My wife and I found a wild rabbit with myxomatosis a couple of years ago. It was wandering about in the road with cars avoiding it. We stopped and took it to Tiggywinkles wildlife hospital which is nearby. They said there was no treatment for the disease as such but they would treat the symptoms and feed the rabbit a good diet to help it fight the disease itself. Apparently, enough do recover for the species and disease to survive. They said most rabbits die quickly but just a few recover. This one died the next day apparently.
As I understand it Iainej, there's no cure for myxomatosis. I think the vets know the disease when they see it so they must be fairly sure about your friend's pet, otherwise they would have tested for other conditions. It's just a matter of keeping the rabbit as healthy and comfortable as possible and hoping it is one of the few that does recover.