Based on our experience with the Lightheart gear duo, I'm a little worried that your dad might end up knocking the inner poles down. That happened with my BF, who was not on shrooms nor herb, nor stiff with arthritis, just being his own sleepy self, so your dad might have even more problems. It's not a tent for the spatially impaired to get in and out of as easily, due to the poles not being firmly held in place. A lovely tent otherwise (sigh).
I was thinking something like the smaller Oware pyramid (or an MLD Supermid) might be easiest for him, having a nice tall door:
It does have a single center pole, but I think those would be easier to avoid than one next to the entrance.
The duomid is nice, but a little tighter to maneuver in than a larger pyramid, and the door is probably lower than the full pyramid above.
I think the Lunar Duo might be very nice. Poles are outside, so that would eliminate that hazard, and I think the poles are anchored at the bottom so harder to knock out of the way. Double Rainbow would also be good, maybe even better since there is NO pole at the entrance.
If you don't mind slightly higher weight, the Double Rainbow/Lunar Duo might be the most hassle-free for him. Otherwise I'd recommend the full size pyramid, I think.