Blocked arteries

This is about my father. White, male, 68 yo.

He has spastic quadriparesis due to spinal crushing from a work accident 20 years ago. Overweight due to inability to do exercise as in an electric wheelchair, although my mother feeds him very healthily. He is mentally sound at least.

He has just has been told that of his three arteries, one is 100% blocked, one is 99% blocked and one is 60% blocked.

They would normally recommned a tripple heart bypass, but they doubt he'd survive it, so instead are considering a stent in the 99% blocked artery. (This is in the UK in case that somehow matters)

So my questions:

1. How big of a procedure is this?
2. How likely is it to be successful, and what would be considered a success?
3. How big of an issue is this? As in days are numbered so start making your peace or could very likely have a good few years left?

Thanks.