I would say... "Wow I love firearms, don't you? Such finely honed instruments capable of extreme precision. Boy, this one time this guy was showing me his carry piece, and he went to hand it to me with his finger on the trigger and blew a hole in the wall behind me. Scared the bejezus out of me."
I think something like that is the nicest and most tactful-in-a client situation way to handle it.
Don't bring up that he had his finger on the trigger, and don't do it when he has his finger on the trigger, but I'm sure if he's a smart guy that hasn't had a lot of firearm training or expirience he'll get the idea.
I've had to do this when a girlfriend's dad showed me his gun collection. Every damn gun he would put his finger on the trigger!
I noticed he didn't do it after I relayed that story.