I'm assuming on PC and solitaire means they aren't close by, perhaps living on their own or a nursing home? Point being = social interaction with others is a step up and engages more of their mind, retains more of their function ability. How old? What environment? How is their mental and/or eye-hand? Is eye-hand something they'd want to work on?
Sticking with PC....this should be my forte, but I'd have to go back and look. There's a tone of freeware out there which is simple by design, therefore relatively (ha! punny) simple to use. I'll go back and think of some candidates, but it would help to know how sophisticated they are looking to get - Role Playing Games (RPG) where they can develop a character and get attached with customizing? A bit more strategy with turn based games? Something they can do in 10-20min and be done, or something they can save and return to later? The old man in my is concerned they are only doing point-click which isn't doing a lot for them physically, but I don't know if that's their current limit. I"m also concerned about the screen and visibility, as the geek in my wants to connect the PC to a large screen tv, but that's likely uncalled for here.
Consider switching to a tablet? Runs the risk of getting left somewhere, compared to the PC which provides a 'place' to go to, and spend time away from. But mobile gaming is flooded with such games to try and discard. My concern here is again eye-hand work is limited, eye strain and visibility, potential misplacement of the tablet, and of course the need to lock it down from spending in apps. But it really is THE place for exploring such games. I'd do a tablet over a phone, primarily for the screen size and findability when misplaced.
I second the other recommendations for board games if at all possible. With family, or in a nursing home setting (my mother in law is in a retired community with card night, I'm sure such communities have board games). The whole point being to NOT sit alone and point-click, but to get more engaged with others WHILE sharpening the mind. Simple starters here could be card games (Uno has a dozen variants, but the basic is still good), or the old classics the person may recall how to play (monopoly & life, also with new variants but old standard versions as well).