My heart went out to you because I found myself in a not dissimilar situation with my Dad. My stepmother couldn't cope with his illness and wouldn't let anyone into the house to help. Eventually, I wrote a letter to their GP and asked him to find an excuse to go and visit them and to judge the situation for himself. I was worried my Dad, who was suffering dementia, was not being adequately looked after - the house was **filthy** and he wasn't receiving enough nutrition. To cut a long story short, the doctor did go and things started to happen. My step mother accepted some help so long as they weren't "social services" and eventually my Dad went into a lovely home.
So, I'd write to his GP expressing your concern and explaining that you don't want to be named for the reasons you give above. He may be able to request your Dad visits for a "routine checkup" or better still, if they share the same GP, maybe the GP could visit the house ostensibly to check up on your Grandad's partner. It's better to do it by letter because the doctor can't ignore a letter and pretend he/she is unaware.
Good luck