jc,
He can mount the stock clip-ons above the top yoke with no modification, all will fit just fine and just barely miss the fairing. It's quite a chore but it can be done at no cost. Is his D12 a 94, 95 or 96? The reason I ask because the 94 & 95 forks extend above the top yoke enough to be able to mount the clip-ons to them. On the 96 the forks are flush with the top of the yoke so you must slide the forks up enough to mount the clip ons to them thus changing the steering geometry by lowering the tripleclamps. I tried this for a while at first on my 96 and found the handling heavier feeling, didn't like it and moved them back.
There are other options like Helibars which work fine but I'm currently using Convertibars (
www.convertibars.com) on my D12 because they are infinitely adjustable. You do have to buy a longer brake line for the mastercylinder and reroute the throttle cable though. I'm very happy with my setup and it's much much more comfortable to me.
Get your friend to check out my photo album for pics of my setup and get him to post me if he has any more questions.
dc