I work with athletes and orthopedic injuries as well, hense the Dr. in the name. With all due respect, a personal trainer does not have the education to make the claims you have made above.
The issue with a meniscectomy depends on the size of the meniscus removed. Small tears are usually able to be played through and fixed in the offseason. The fact that he had to miss time immediately tells me it was big enough to cause swelling and pain and needed to be fixed. Possibly a bucket handle tear. When you remove the meniscus you essentially are removing a layer of cartilage that protects the articular cartilage of the knee. This fast tracks knees toward OA and other issues, such as more meniscus tears or ligamentous strains, as the surrounding tissue is strained more.
I'm glad to hear your own knee surgery went well, but as you stated above you are not an NFL WR. So your experience is very different to Gallup. I have treated current and past NFL, Olympian, semi pro, college athletes. I know what I'm talking about here.
I wouldnt worry about his knee within the next few years, depending on the size of the tear, but long term I have concerns. I do own him, and I will probably try to flip him for a pick when I can.