In reply to Nutkey:
Heading from the North Face car park up Carn Mor Dearg, then round the Arete on to Ben Nevis, then down again, is all very straightforward technically - just a little very easy scrambling at a couple of points on the Arete. However, a lot of the ground is very blocky and jarring, and you're starting from near sea level and going over two Munros - about 1500m total ascent.
Quite a number of 70-year-olds would have no bother with this, but a lot would find the descents, in particular, taxing. Probably the worst section for this is the Ben Nevis side of the Arete, and the slopes above that, so I would aim to be going up those rather than down (so doing Carn Mor Dearg first).
Just doing the Pony Track from the car park in Glen Nevis (the normal start for those walking up the hill) is substantially easier in this regard, but it's a much less interesting route, so it depends on your Mum (and you).
As for when you should start, well - when do you want to finish, and how fast do you expect to be? How long have you taken to do something we can use as a benchmark? It isn't going to get dark until very late, and even then it won't be very dark unless it's overcast, so time limits are really down to you. It'll probably take you between 6 (very fast) and 12 (quite slow) hours to do the circuit via Carn Mor Dearg, but even that's a guess.