It happens to almost every amateur pilot. Professional pilots too, if they are not subject to regular checkrides.
In 1983, Embry-Riddle Aeronautical University conducted a study designed to (1) measure the skill retention levels of newly licensed pilots and (2) determine how accurately these pilots were able to predict their own level of personal proficiency. I can't find a link to the study results, but they are summarized in Richard Taylor,
Pilot Profinciency (1991), at pages 8-12.
The study revealed that general aviation pilots suffer a significant loss of cognitive skills (i.e., pilot judgement and decision-making ability) and flight skills within a short period of time following the completion of structured flight training. The areas of flight skill loss most affected included takeoffs and landings, stall recognition and recovery, minimum controllable airspeed, and emergency procedures.
Pilots' ability to predict and evaluate their own skill retention levels for specific flight tasks was found to be negligible.
How do you go about telling a pilot they need some dual time, when they don't or won't do so.
Obviously I know nothing about your friend, but: why do you assume that he will be unreceptive to your feedback? PResumably he is not entirely unreasonable (given that you consider him worthy of your friendship), and obviously he has a vested interest in increasing, or at least maintaining, his proficiency.
Personally, I only fly in club settings. I find that the atmosphere of mutual support and constructive criticism helps keep me safe. I undergo regular checkrides and, when not 100% up to par (yes it happens, especially after a long winter of no flying, etc.), I don't mind suggestions that one or two dual training sessions might be appropriate.
A lot depends upon how tactful one is. "You know, I noticed that your speed control during the landings was a bit rough. Why don't you book an hour with Charlie next week? A few circuits with him would do wonders" is more likely to be well-received than "You're bloody dangerous! You should be grounded!".