We live in southwest Tucson for the winter and the Pacific Northwest for the summer. I have a mountain bike and a road bike in both places. My wife has a mountain bike with city tires in each location. We are over 65 and don't ride clipless.
In Tucson my wife likes to ride the off-road paved trails along the Santa Cruz. She can't figure out how to shift the triple rings and does not have a great time. I should have got her a light hybrid without the front suspension. Her friends ride the Rillito river trail.
We have Robles Pass of Tucson Mountain Park at our back door so I spend a lot of time riding trails.
My road rides are south on Mission to Green Valley and from Green Valley to Arrivaca when I need a change.
Amado to Arivaca in Green Valley, AZ, United States | MapMyRide
Green Valley to Madeira Canyon when I need to climb.
Green Valley to Madeira Canyon in Amado, AZ, United States | MapMyRide
If I was your son I would try out different types of bikes at Performance Bikes to see what works (try PB where he lives now). I would probably look at light hybrids. If they are going to be riding together it is important that his wife is comfortable and enjoying the ride. If they get the bikes on sale they can probably sell them on CL for what they pay if they decide to upgrade after riding for a while. Did I say that it is all about making sure that his wife feels safe, is comfortable and is enjoying the ride?
They will like Tucson. They may be able to bike to work. You will be able to visit in the winter.