With surgery, it may be preferable to take higher dose Viagra for on-demand use than to take it every night prophylactically. The reason is because ED meds have a stimulatory effect on the urinary tract, which runs counter to what he's trying to achieve in terms of continence.
Nightly sildenafil use after radical prostatectomy has adverse effects on urinary convalescence: Results from a randomized trial of nightly vs on-demand dosing regimensOn the other hand, nightly use promotes early recovery of erectile function, and helps prevent loss of penis size and surgery-related Peyronie's. It may have a negative impact on climacturia.
I hope his doctor will start him on a full program of penile rehab as soon as the catheter comes out, including frequent use of a vacuum pump, and trimix injections if ED meds alone aren't enough. Early use of penile rehab often improves all those things more than waiting.