At that age surgery is just not an option, too hard on the body. Many doctors won’t even consider it if you’re in your 70s.

Radiation works quite well, the question is how bad the cancer is. How high is the Gleason score? If it’s 3+3 no need to do anything yet. Active surveillance is what to do.

If the Gleason score is higher how many cores out of the biopsy had cancer? Were they higher than 3+4. Did you get a decipher score?

Have you had a PSMA pet scan and has the cancer spread out of the prostate?

All of these are things doctors consider, age is just a factor, radiation might be perfect or might make no sense, yet.

Have you considered going to a mayo clinic? There, doctors can get together and help make a decision on what is best for somebody who is 89. All the issues I mentioned would be considered, and probably more.

I wish I could tell you there's a best alternative, but there are too many "it depends" involved. Radiation and surgery have the same outcomes for overall survival, but bring different side effects.

The first question for your friend to ask is how advanced it is and how aggressive. Since he's 89, if the cancer is still localised and has low scores for aggressiveness, your friend's medical team might just suggest keeping an eye on it without any major interventions.

Can’t say yes or no, but surgery should be completely out of the question in my opinion - too risky at that age. treatment would depend on many factors: Gleason score, Decipher score and most importantly your friend’s general health.
At 89 there are usually medical conditions that can weigh heavily on life expectancy, etc. He may not even need treatment depending on all of the above but he and his MD really need to have an honest discussion. Best to him.