I think this is a potentially dangerous activity for an out of shape 450 lb person. For some reason, people push themselves hard when they walk up stairs. Normal stair climbing is like running. I don't think a totally out of shape hyperobese person should go out and do anything intense right off the bat, like climbing say 5 flights of stairs. They could easily fall over dead. It might not happen but it could.

I would think that they need to go to a physician first and get thoroughly checked out and by a physician with experience with morbidly obese people.

Also any exercise program they do undertake should be pretty low intensity at first and hopefully not weight bearing.