There is a "science" behind the level of PSA decline after surgery...I'll explain how it works. It depends primarily on one's PSA prior to surgery...the starting point. Once the primary source of new PSA is removed (the prostate and seminal vesicles), then the PSA already in one's bloodstream diminshes over time.
Studies have been performed which show a distribution of how fast PSA diminishes, but in most men the "half-life" of PSA in one's blood after RP is 2-3 days. So, if one's "starting point" at the time of surgery is 5 ng/mL (close to yours, Larry; and about twice your friend's PSA, but you can easily see where his fits in, below), and if one assumes a 3-day (the slower assumption) half-life, and if one assumes no additional source of PSA added into the bloodstream, then the PSA level would reduce like this:
5.0 ng/mL at time of surgery
2.5 ng/mL 3 days later
1.25 ng/mL 3 days later, 6 days after surgery
0.625 ng/mL 3 days later, 9 days after surgery
0.3125 ng/mL 3 days later, 12 days after surgery
0.15 ng/mL 3 days later, 15 days after surgery
0.08 ng/mL 3 days later, 18 days after surgery
0.04 ng/mL 3 days later, 21 days after surgery
There would be some statistical "outliers" to the 2-3 day typical half-life which would result in a longer (or shorter) time to diminish...so add a week or two for "margin." This is the science behind why surgeons prescribe the first PSA typically 6 weeks after surgery (some very conservative doctors want to wait longer...the longest I've heard of is 3 months).
Of course, this half-life decline described above assumes no PSA is being added back into the bloodstream. There are, however, secondary (minor) sources of PSA, however, which prevent any living, breathing man from every achieving "zero" PSA...the phrase "zero club" is a misnomer which sometimes causes undue worry in men when they hear that their PSA is something other than "zero."
A common source of PSA being added back into the bloodstream after RP surgery is the neurovascular bundles which may have been safely left behind during surgery; they do produce a low level of PSA, and the amount produced varies from man-to-man. The PSA produced by the nerve bundles after surgery is not a sign of cancer, but it can be a measureable/detectable amount of PSA in the bloodstream...depending on the lower detection limit of the PSA test which one chooses.
Prostate cancer also produces PSA, and so IF PC cells were left behind after surgery, they would also produce an amount of PSA proportional to how much PC was left behind. Obviously, if PC was left behind and the cells start to multiply, this would lead to an increased production of PSA in the blood stream...which is why we continue to have PSA tests after a RP monitoring for increases. PC cells produce about 20x the rate of PSA as healthy prostate tissue cells.