Well having 25 years of experience I would disgaree with much of this. I am willing to bet he is overtrained. The amount of volume is different for beginners. If he is light or trying to gain weight, he could easily be overtrained. If he is a bigger man, trying to lose maybe not so much and he could be at a mental block. No offense but I could not possibly believe any good bodybuilder especially not any powerlifter worth a grain a salt will ever tell you benching 3 times a week is good to gain size or strength.
That once every ten days is an old time trick. Its not a permament one, but it would definately help if its a mental one. I gave some advice based on many many years of training. I am in my 40s with the body of man 30. I have had the good fortunate to train with a couple state level bobybuilding champions, several top east coast powerlifters as well over the years.
Its hard to give good training advice when you don't know the person face to face. As I said I wouldn't lighten the weight, I would suggest a couple less sets or change in routine to get away from a possible mental block.
Let me also add this one note, I could see a bodybuilder benching 3 times a week getting ready for show, but he won't trying to move more weight like this young man here is trying to do.
While I respect your years of experience, it doesn't trump 25 years of research. The human body isn't helpless. There are many programs that have had people completing the same or similar lifts 3x per week without any problems for years. I don't know if you are using the official term of overtraining, but that takes 1) significantly more time with weightlifting, 2) isn't really plausible here due to the intensity and slight stall. If he were truly overtrained, he would be noticing declines in ability to sleep, resting heart rate, he may actually regress in weights lifted. He's just talking about a plateau, which in beginners is almost always a motor recruitment issue (not optimal motor program/form).
The old bodybuilder montage of doing one lift once every week is based on false knowledge of enzyme biomarkers. Creatine phosphokinase peaks and then trails off about in that time, but on the large scale, people can lift every other day (with intermittent breaks every few months) without problems. Also, keep in mind that the demands for a good, experienced powerlifter or bodybuilder are significantly different than for a novice. Of course they wouldn't say that. However, if you want to go that route, Louie Armstrong from Westside Barbell frequently has people doing the same lift twice or even three times per week.
I agree that it is difficult to understand the problem fully without knowing the person, but research doesn't support that lifting LESS frequently helps. He could even just modify one day into a dynamic lifting day for bench where he did 10 sets of 2 at a fairly light weight. Or he could do something like Grease the Groove, both of which are focused on improving neural coordination. I bet that would give better results than reducing his benching frequency. It also has a basis in research.