So, the OP's father is 70, uses 10 year old Volkl skis which would probably be around 75mm waist width, and has "slowed down." He skis 90% groomed, and occasional sparse trees, which means he is not skiing ice, moguls, soft snow/powder or steeps to any extent. Nor does it sound like he is just aching to join the pro mogul circuit, jump into Corbett's Couloir, or go heli-skiing in the next few years, it sounds like he is perfectly happy to continue to do what he is doing. In other words, a ski of the same width as what he is (probably) using now is probably the best choice for his new skis. Basically, his profile fits much better with a frontside ski than an all-mountain ski.
The SeniorSkiing website lists recommended skis in collaboration with RealSkiers. Their current list of frontside skis includes the Volkl Deacon 76 and Liberty V76. The Stockli Laser AX is also on the list if you want to go upscale, and the Blizzard Brahma 82, Dynastar Speedzone 4x4 82 Pro, Elan Wingman 82 Ti and CTi are also on the list if you want to go for a little bit wider ski. Note that the Elan Wingman skis are asymmetric, so if he is used to periodically swapping right and left skis those would not be a good choice. That should be a more than adequate list for demo.