Hi Mark,
You made a good move asking for advice although it probably needs a deeper discussion than a forum like this. I'd suggest you personally speak to Macmillan, AGE UK or other similar group in your area. The local hospital may have a Macmillan suite. They're all very sympathetic and want nothing more than to help.
Without wanting to put pressure on you, as you're obviously caring, you could play a big part in re-aasurance withiut overdoing it. I recall thinking that getting Prostate Cancer was a bit like the bereavement process where you have denial, anger, bargaining, depression, acceptance. It's important not to get stalled in any phase. It's amazing that just about every human goes through these phases and it can be quick or slow and it's worth looking into. There's plenty on the internet.
There might be other things about your mother and it could need a long discussion. Perhaps someone else will also suggest something as we all look for different things. All the best. Peter