Most wind surfers don't sail under 15k i don't believe? So any kite around a 15m/17m will do light winds up to 15k with ease. Board go some thing around a 145x45 i would not go any smaller and flatish minimal rocker.

It doesn't really work that way. You don't just grab a big kite and go kiting. Light wind kiting is an acquired skill and skills take time and practice. Once you have the skills you can ride in any wind with any gear and make it work. Without the skills you are just fighting your gear all the time.

Your mate should go through the basic learning phase with mid-range gear, get the skills, then specialise from a position of knowledge. As you have hopefully worked out, to be successful in kiting you need to develop the kite flying skills to a point where you can do it without thinking.

At 80kg my light wind kit used to be a 13m bow kite and a 135x48 TT. I now use a either a 10 or 13m bow and a very flat surfboard shape, or a freeride raceboard (Airush Sector 60). The Sector is a very similar experience to windsurfing.