I can't think of any reason why a 79-year-old shouldn't have the op. Reliable internet sources (i.e. those based upon scientific studies, rather than one individual's view of things) state, for example, that one in 8 TURP patients in Austria are over 80 years of age.
It's one of the most frequently carried out operations in the UK and, from the patient's perspective, remarkably simple. (Mine was carried out with a regional anaesthetic, meaning that I was wide awake all of the time that the surgeon was snipping bits out from inside me. Both the operation itself and the recovery period were totally pain-free).
There are, however, alternative procedures available, which are mentioned here:
https://www.nhs.uk/conditions/transurethral-resection-of-the-prostate-turp/
An enlarged prostate can, of course, be a sign of prostate cancer. (My TURP operation found cancer cells, whereas my friend's didn't). So, irrespective of what type of treatment might be offered, it's important that the person you refer to gets checked out by his GP and/or a urologist with, at the very least, a PSA test every so often.