In reply to alps_p:
All good sensible answers to your question, just a couple of points that I would add.
The technical sport rock grade he climbs is almost irrelevant, nothing you are likely to attempt this trip will come close to it. As you will already know, the Hornli ridge is mostly a long diff with a few bits of V Diff, but that does not minimise its seriousness, which remains substantial. The diff bits mostly have to be more or less soloed, so he has to be happy moving on easy but serious ground for extended periods.
Obviously it is worth having alternative routes, to deal with bad weather, mental wobbles or whatever. Never get too fixated on a particular route in the Alps, adjust to what conditions allow, so you at least come away from the trip with something worthwhile. I try to have a full range of fall-back options mentally worked out, in case the main targets are not suitable for whatever reason.
You may find that he is more un-nerved by extended amounts of loose broken ground or the general impressiveness of the Alps than any technicality. Last year I was climbing with a relative novice who certainly was technically strong enough but was un-nerved by a steep, loose and unpleasant descent where we were never very safe. Fortunately he had the courage to be honest with me about having lost confidence, so we were able to adjust our main objective from something quite demanding to something less committing, but still on a 4000m peak - basically just not ready at that point in his climbing career for serious Alpine exposure. Your mate may feel the same, or may take to it like a duck to water, but the overall sense of seriousness in the Alps takes most people a while to get comfortable with.
So try for some warm up routes were you can easily back off if the brown stuff hits the fan, where the situation is relatively controlled. Then work up to more challenging routes if it seems to be going OK.
Post edited at 14:55