What is the best way to make money without physical work in Real Estate.?

My friend He has severe back problems and stays home all day but have lots experience in real estate he used to to buy rental properties and owned for a while and then sold them for some profit, But now it is not possible to do the same . Iis there any place in Real Estate he could make money without any lifting or repairing properties. I sugested Tax Lien certificates but I dont have any clue about that myself if it is the best. What do you suggest where can he excel as investor. I shall highly appreciate