Talking heads are paid. If it's a public figure, a fee for his time wouldn't be be unreasonable. I think the question would be whether your mate's willing to pay him. If he's willing, and the subject will accept, then hey! As long as the filmmaker's happy, and is willing to defend it to the death, I don't think it's a problem.
I'd offer without money, first. If the subject says "what's in it for me?" then counter with publicity. If he says "sorry, that's not enough," then I wouldn't hesitate to get a big fat wad out if it'll facilitate the production of a good bit of telly. No shame in it, though. I've produced documentaries where contributors have insisted on payment, and ones where they haven't. Doesn't matter what anyone will "say." Your loyalties lie with whatever makes a good programme, not with a persnickety gobshite on the internet who's on the hunt for a conspiracy. Money will not compromise your friend's vision.
I think a bigger hurdle would be that the subject, if as particular as you believe, may not be willing to entrust his public image to the hands of an outside production company with no stake in his own image.