RE: Which second board for an F-Type 138 owner?
Using the RS6 7.2m will be OK with the C111, especially when set up with outboard (semi slalom) mode straps. Using a slalom fin on these boards is definitely possible (and normally with an RS would be a good combo/option) but it definitely stiffens up the ride/handling, and detracts from the natural jibe character the Carve is legendary for. In this case I wouldn't rush to recommend a slalom fin until your friend enjoyed the benefit of the easy jibing nature of the Carve with its freeride fin - obviously an easy and secure, progressive jibe will help him improve his jibes (and confidence). And fun factor.
Obviously, having a good freeride sail would make an even better match (and an easier progression in skill/fun etc) but if he insists on the RS, well at least have a few good sessions on the freeride fin before converting the C111 to a demi slalom rocket with the race /slalom fin.
Cheers ~ Ian