I think either gun would be just fine for home defense. I can't imagine any of his shots would be at a terribly far distance. I'd say have your friend go with whichever one feels better in his hand as well as which he shoots better.
+1 to this. I'm going to interpret "mechanical accuracy" as to mean which one groups tighter out of a Ransom rest, all variables of distance, rounds used, etc being equal, which won't translate to anything all that useful unless you're friend's planning some kind of strange home defense set up . . .
As infectednite said, it's much more important how the gun shoots for him, that is going to determine which one is more effective for him, and ultimately allow him to be more accurate. Barring some mechanical defect, they're both going to be more than precise enough for HD applications.
As far as for his wife to shoot with at the range, he'd be wiser to involve her in the fit and selection process (i.e., have her shoot a few) to ensure she gets a gun that works for HER, rather than picking a gun for her that HE likes, which may not necessarily end in her wanting to shoot it. Or being comfortable with it as an HD weapon. Is it a sidearm for HD for him, or a gun for her at the range? (Best solution, buy two!

)
Can't count the number of times I've heard or read "shoot as many as you can to find the one that fits you" and yet so many guys seem to think the better way to pick a gun for their wife/girlfriend/significant other is to pick it for her, based on what they like.
That said, the XDm's interchangeable backstraps may lend towards a better fit than the 92 if she has smaller hands, and switching the backstrap up for his hands if they're bigger will help both of them effectively shoot the same gun. And swapping backstraps in the heat of an HD situation - :shock: - won't be an issue,
at HD ranges, IMO the difference in the sizes aren't that much of a difference. Results may vary, so trying out the different sizes would be wise.
Good luck, have fun helping your friends shoot a few!