Fcuk 'em, mate. And fcuk feeling sorry for old people.
Just run off and leave them to it.
If you do it properly, then you, them or their son in law will feel bad because it was expensive and if you somehow cheapen it up and it goes wrong, then guaranteed the son or daughter in law will be ringing up to say that it's not a very good job.
They have got the money, got a house, haven't they? But they'll all have been watching cowboy builders and be looking out for Ole Nan.
I got absolutely hammered years ago under just these circumstances. An old woman had a problem - turned out she had no lintel in a little coalshed thing that she wanted to turn into a sort of porch to sit in. I felt sorry for her so I said I'd put one in and dash for £180.
Except what little lintel there was went into the house by about six feet and only a foot over the window opening, then I couldn't find the fcuking dash anywhere. When I did, after two days, it turned out to be the most expensive dash that money could buy.
The long and the short of it was that what should have been a day with £60 materiels turned into a week with £50 profit.
Just don't do it mate.