I would probably recommend against getting a dual VHF/UHF antenna and instead think about adding a UHF antenna to the VHF one that's in place now. I don't know much about the Antennas Direct model you mentioned, but I see that it's multi-directional. To have even a shot at getting KVLY and KXJB, a better option would probably be to get a purely directional antenna, and the biggest, most directional one you can find at that. You also don't really need a multi-directional UHF antenna since all the UHF stations with the biggest chance of coming in are all located in the same general direction. The Antennas Direct 91XG or Winegard HD9095P would be a couple of options. if you point it just slightly to the east of due south from Hoople, you should have a good chance at picking up PBS station KCGE on channel 16 and you might have a shot at getting KVLY and/or KXJB. Northern Walsh County is really at the very edge of the KVLY and KXJB broadcast areas, though. Still, if it's any consolation, I've observed firsthand that TV Fool is a touch too conservative with the predicted coverage areas of those stations. The computer model probably underestimates how powerful those stations are with their very tall towers and the incredibly flat terrain of the Red River Valley.
Now, as for the VHF antenna, if what you have now is a directional design, I would imagine it's pointed to the south in order to pick up all the stations that used to broadcast on VHF. If that's the case, I would try to flip it so that it points to the north-northeast. Although not listed on the TV Fool chart, CKY, the CTV station in Winnipeg, broadcasts a digital signal on channel 7 from a site near Ste. Agathe, MB. Hoople is about 75 miles by air from there, and, like KVLY and KXJB, right about at the edge of the station's coverage area. But it can't hurt to try for it. The antenna would not be aimed in the right direction to pick up WDAZ, but that station is close enough and strong enough that orientation of the antenna might not matter as much. Picking up CTV would definitely be a bonus, especially if you are unable to get CBS (KXJB) and/or NBC (KVLY), since some of the shows your dad might be interested in watching on those networks could be broadcast on CTV.
If you do decide to go the route of a separate VHF and UHF antenna, I would also recommend a preamp just because you'll absolutely need all the signal you can get for the UHF stations and CKY. You can get a preamp with separate VHF and UHF inputs to accommodate two antennas.