If you're selling it soon (which you're not) then best just to sell it, as say £30k of renovation doesn't usually translate into £30k on the selling price. The people who do manage to see all their doing-up costs more than paid for by increase in selling place are builders who are paying trade prices for goods and have reduced labour costs, and accomplished diy-ers who can do most of the work themselves.
If you're not selling, there are various costs to think about. An unoccupied house may have a period of grace but often is liable for double council tax. Insurance is more expensive because there isn't someone in the house to take early action in case of problems, and usually requires you or someone else to inspect inside and out every two weeks.
How long do you think before you are in a position to sell it? The average stay in a care home is around 3 years, less in a nursing home, but of course people can be in a care home for a great deal longer - 12 years, possibly. If you think it will be a long time before you want to sell it, you will need to do work on it to maintain it, and keep the garden tidy, etc. If you do maintenance on it, in my opinion (as a layman) it's reasonable for your grandad to pay his share, because it's necessary maintenance on his asset. I think you could argue that for boiler replacement and re-wiring. But I wouldn't use his money for anything that's not strictly necessary for maintenance and wouldn't therefore benefit him. You might want to take professional advice on this.
We can't answer whether it's best to sell up or keep. That depends on such things as - would one of you want to live in it, would you want to rent it and how much would it cost to bring up to the higher standard required for renting. How big a house is it? - if it's a 5 bedroom pile, and you are a single person on an average sort of wage, you might struggle with ongoing costs; it it's a 2 or 3 bed, it might be a very sensible move. Nowadays it often costs more to rent than you would pay on a mortgage, and you have a house at the end of it.