They don't need another television. Any advice?

Give him back the old TV, this one has him totally baffled.

Maybe planning for a new tv could be his current project. Where would they put it? Walk around the house with him, seriously considering the placement. Talk about ripping out walls, moving furniture ... the sky's the limit. This is fantasy anyway. Go through newspaper ads (or online, if he uses a computer) and get measurements and costs. Compare values. Compare costs to something he can relate to, like the cost of heating and cooling the house for a year, or how many months of groceries that money would pay for. Frequently divert him onto another topic. "What is your favorite show to watch on television?" "What do you like best about that?" "What was your funniest real-life adventure?"

Talk this project over with his wife first. Tell her you'll deal with it so she doesn't have to. The point would be to give him something to think about and plan, to generate topics of conversation, to keep him interested. Don't argue with him about it. There can always be reasons why you can't act immediately ... the contractor for tearing out the wall is booked all season, the appliance store is closed for a family funeral, it is going to take a while to save that much money, etc. You aren't going to actually buy an unneeded tv -- just keep him occupied without fussing at his wife.

The exact nature of his dementia will make a difference in how to approach this. But I'd try for respectful interest in his ideas and no arguing.

It seems the largest size would completely be overwhelming in most homes unless you have a theatre room or something. Could he be wanting something smaller instead of this huge white elephant? If so, maybe kids will replace it with something smaller.