Hi Leo, I think we're talking about the same lens. It is the original AF 70-210 with a fixed maximum aperture of f4 at all focal lengths. It was only made between 1986 and 1987. It has two rings where the other, newer, versions are single ring push-pull operation. If your friends lens works with the Kenko TC then that's fine. But it's got to be really close to touching. Before I try a TC with a lens that has a rear element past the lens mount I always measure the clearance distance. In this case I found it too close to try. I still won't try it. The lens is becoming hard to find for a good price. Mine is in nice shape.
I also have one of the newer AF 70-210mm f4-5.6 versions. It's push-pull single ring and there is no problem mounting the Kenko TC. Actually it works pretty well.