Can we just be clear. HRT stands for hormone replacement therapy. It is a treatment for women on the menopause. HT stands for hormone therapy and can mean almost anything where hormones are manipulated, including HRT (but never for prostate cancer).
ADT stands for androgen deprivation therapy, and this is used for prostate cancer treatment. As it is achieved by manipulating hormones it is therefore a form of HT. It is almost the opposite of HRT as you are trying to deprive the body of hormones not replace missing ones. Most people on this forum use HT and ADT interchangeably.
Now me being blunt, in an 80 year old, you only need to hold off prostate cancer for five to ten years. With G8 refusing treatment would probably end in death within three to five years, and not a very pleasant death. RT alone would probably have a 50% chance of a complete cure, and even if not a complete cure would probably set the progression of the disease back by five to ten years, so RT alone is a realistic choice. HT (ADT if you prefer) tends never to cure on its own, it holds the progression of the disease back, but usually fails in about five years, so not really a long enough timescale for your dad.
RT and HT together make the chance of a complete cure about 70%, and if the cure was not complete would set back the progression by about five to ten years, so a better option than RT alone and a much better option than HT alone.
The side effects of RT are not usually very serious, they can be serious but it is not too common. The side effects of HT seem to vary most men find it bearable, but for some it can be unpleasant, if your dad was on permanent HT he would probably get osteoporosis after a few years and then other complications. I suspect he would find after a few years the HT wasn't a great choice, and then it would fail and the cancer return.
In my opinion the RT has the least side effects, and if he is currently coping OK with HT he should probably go for RT and HT together, and if the HT becomes unpleasant stop it after a few months, most of the benefit of HT is a few months either side of the RT.