So could you.
At 80 just doing anything other than surgery is good.
Some suggested crunches various clinics etc.
But in reality old dude should just lie on his back and try to do a situp.
By the time he can do one he's on his way.
Later he can get on with crunches, dead lifts and other exercises.
Silly suggestions are just silly. Maybe he should go spend his money here, no go there, no go pay for yoga classes...
Maybe he should do crunches (kind of like trying to do a situp) dead lifts, or squats, or this or that.
It's basically step by step or straining gut muscles then stretching back muscles then more sophisticated stuff.
The old dude should just do something simple and free first.
Enough silliness already.