It is now going on mid Feb and surgery not done yet? If so, it is going to be really tough sledding for him this fall. The recovery timeline will be right at typical hunting start for begin of shooting for him. I have had 3 surgeries on rt shoulder, last one 4/1/21. Prior one 12/18 where the rt shoulder was completely rebuilt. All 4 rotator tendons torn, bicep tendon displaced, bone cyst, torn labrum and bone spurs. It was easy 7 months before I could shoot and it was .243. I tried 300WSM and 300WM but it was not tolerable at all. It was 9 months before I shot my 30's.
I am 71; 6'4" 255 lbs so I have plenty of mass for recoil but even then, it was July before I could shoot after the last surgery 4/1/21. I pushed it a bit since I was building a rifle during recovery (.270TH) that I couldn't wait to shoot. Turned out fine, little ouchy but very tolerable. BUT I believe my body mass had everything to do with it. I was shooting my 30's by end of August.
If your Dad is of slight build, the recoil absorption will be an issue IMO. The older you get, recoil seems to creep up on you as well. I have good friend at 76 that dropped back to .270 cause recoil was causing horrible flinch. He is of very slight build.
Lots of suggestions here but his age, body mass, surgery timeline, actual damage repaired, recovery timeline will all be major factors in not only when he will be comfortable to shoot but what cartridge with comfort. Older folks can be more frail than others and need to adjust their hunting "styles" to accommodate facts of life getting old. I can shoot my 30's but I prefer my .270 with 156HH. It just seems more fun for me to use it.
Option I would suggest is the 7-08 with 90 gr Hammers that will reduce recoil immensely, be laser, kill DRT especially at the range given.
Good suggestion of PAST recoil pad, I use it and definitely reduces felt recoil.
LINK: PAST recoil pads