[Yoda]

A friend I play with has, what I now know is called, a severe 'anterior rotation'.

When his arms hang naturally, the backs of his hands are parallel to the target line. (I promise, he does have opposable thumbs.)

What do you feel would be the ideal grip for him?

I understand that PGA TOUR player Tim Clark http://www.pgatour.com/players/02/31/35/ has a similar problem. In his case, an elbow deficiciency severely limits his ability to rotate his Left forearm, so much so that, as he says, "I can't even get my change at the drive-through". In his case, he just goes with the flow and plays with the super-strong Turned Left Hand. Seems to work!

I'd have to see the situation in person, but it sounds as if your friend should do the same. By the way, there are specific exercises that can help immensely to reverse the condition and produce physical benefits far beyond swinging a golf club. We will introduce them in the forum Fit For Golf With Vickie Lake. I know she can help us here:

I'm on the program!