If they are worried about your grandad's heart they won't put him under general anaesthetic, they'll give him an epidural and sedate him most likely. This puts less strain on the heart and your grandad will be awake for the operation, although mostly unaware of it. My grandma (86) has just had this done for similiar reasons and came out fine. She's been up and walking today! I also work as an auxilliary nurse, sometimes with hip patients and i'm pretty sure that's what they'll do if it's risky. If they give him a general anaesthetic they'll be knowledgeable of the risks and confident that they're not great, so although it's scary i wouldn't worry. He really is in the best hands.
I hope your Grandad copes well with his operation and enjoys walking properly again. xx