You never know how you're going to do until you try. Your friend might surprise himself. I've signed up for a 100K on my single in June. The ride has about 6000' of climbing which I believe I can do having done 4000' or so on the tandem. I'm considering the 100mi ride with 7800' of climbing, but this is not a fully supported ride and I could be toast in the middle of nowhere; we'll see what unfolds. None of us want to except limitations as we get older; why should we?