While my husband doesn't have stomach cancer, he did have 35 radiation treatments on his neck for laryngeal cancer. The worst part for him was not being able to swallow after 2 weeks or less of radiation. He did have a feeding tube inserted before starting the radiation.
Now we are getting ready for a second round for esophugus cancer. Whether he can have more radiation around the area of where the tumor is, we are not sure yet. My husband had said he wouldn't undergo radiation again, and that is what we thought he would say this time, not true. He is ready to go the full route of treatment. I don't know how old you father is, my husband was 73 when he had the first round of radiation and is now 75. I met people younger and older undergoing radiation and all had different stories since no two bodies react the same.
You might want to get another opinion on the radiation. We were told that radiation on the neck was the worse and that many didn't complete the course. The decision to undergo or not is your father's and many have turned down radiation and had no problems not having it and others have problems.