The FD shifter on my 105 ten-speed triple broke and I replaced it with a 105 double. I didn't know that the 105 STI came in a double-only configuration, but it does. And that's what I bought on Ebay. So, I removed the small chainring and ran it as a double. It worked fine. I mostly stayed on the 39, which I think lines up best with the cassette, so not much cross chaining even at the extremes. Now I'm using a standard double and I really like it. So I guess I'd recommend the double or compact approach, but I don't think your friend would have problems converting the trip to a double. At least I never did. And yes, I would get onto the big ring on descents; never dropped the chain.