It might be easier said than done, but have you considered just saying "I'm sorry, but I don't feel comfortable helping you with this."
I used to be a dealer, and someone from work wanted to see about buying a gun. I asked him what the purpose of the gun was, and he said for home protection. He then made the statement that 'If anybody breaks into my house, they are dead.' I asked him even if they are clearly surrendering? 'It doesn't matter', he spat, 'they are in my house so they are dead!' I had to tell this young man that with that attitude, he was better off without a gun.I know man, see, this is what happens all too often: the OP asked a legitimate question and it quickly started going south. It turned into a deal all about judging and interfering and so on. Sigh...I do think 65ny offered the best answer to the question so far. Many of us have had similar situations come up, maybe not about guns, but then again, maybe so. Example, a lady my wife and I know has asked me to help her buy a gun and then teach her how to shoot so she can protect herself from her allegedly violent ex-husband. Right. Like I'm going to get involved with that mess. No way. I can see it now: "Yes your Honor, I shot him in self defense, I double-tapped his azz and then dumped the rest of mag in his noggin just like Mr. Druryj taught me to do".
People like these are our worst nightmare. They are how a good portion of the media and the other gun-grabbers see us. If you know someone like this, please discourage him from getting a gun.