Is it an end ileo or a loop?
Firstly, I am sorry your dad is going through all this and I am thankful the surgery is able to give him a chance. I also wonder if it's permanent or temporary? I think that was the biggest relief to me, knowing it was temporary. After living with the end ileo for a while, I felt okay about
it. It was really low fuss and not too much trouble. The loop, on the other hand, was a lot of trouble for me, and not something i felt comfortable with at all. If it's temporary, keep reinforcing that.
Regardless, does he have a nurse scheduled to help him learn? Once he is out of the hospital, your mother shouldn't need to help him, not beyond the first few changes (if he wants help). I would have been extremely uncomfortable having my significant other around while I was changing. I didn't even want him around when I was meeting with the nurse. I wanted to learn and do it myself in privacy. It's like wiping your bottom, do you want someone else to wipe it for you if you can do it yourself? I think getting him confident, and teaching him how to do everything, will make it less scary. It's going to take time.
There is an board for ostomates here on HW, but there's also a huge one on the net, and lots of resources for finding local support groups. It is really difficult to cope with, especially when it's unplanned and a shock. Let him know that there's a learning process and that it gets easier, also let him know that it's okay to seek counseling if needed.
https://www.ostomy.org/support-group-finder/https://www.uoaa.org/forum/viewforum.php?f=2