In addition to not being interested in plastic stocks, or sabots, or those dark side guns, most of us consider ML hunting to be a skill sport, that requires you to actually get in range of your gun before you shoot. Its not a shooting sport, where you simply wait for something to get within range of your gun! We use a PRB and open sights because it requires us to work harder for the deer we kill( sometimes.) And " Long Range " shots at any deer are NOT in the planning.
Since you need a gun with a fast Rate of Twist to shoot those pistol bullets wrapped in plastic, you generally have to go over to the dark side. I have not been, and don't intend to go. I doubt you are going to find many recommendations from people on this forum to answer the question.
Are you sure your father has just not shared with you one of those " fantasies " that even he understands he will not ever achieve?? There are ML rifles made to shoot conicals, that can be used well past 100 yds. But they are not set up for sabots, or copper jacketed pistol bullets. And, we see a lot of mule deer killed each year with standard, Traditional ML rifles using either PRB or conicals. I understand the mule deer have now migrated East and well down on the flats, as they are now being seen and killed in Iowa. I would think your father might have success on a mule deer hunt with a traditional ML rifle, if you match the terrain with his physcial condition.
For instance, My back was out for about 12 years, and I had to stop even thinking about hunting. And, my knees have not been in very good shape to be climbing hills in Colorado since I was in College( where I injured one of them). A mule deer hunt for me would have to done on the flatter land.