If it's at the bone on bone stage, I'm thinking she is definitely a candidate for immediate replacement.
Have "heard" that the replacements are only guaranteed for x# of years and that usually they like to wait till someone is older to do it, but who knows how much real current info the nebulous 'they' that said that had.
One friend got both of hers done as an 80th birthday present to herself. Her analysis was that rehab was no picnic, but it wasn't any worse than what she'd been putting up with for years.
Another friend who is probably late 40s has had lots of problems. Different surgeon, different hospital, different person, different outlook on life in general...who's to say what made the real difference. (I'm betting the attitude on life in general had a lot to do with it!)