Handing a stranger a loaded gun is always a worrisome thing. As a Pistol Instructor and Coach, I do this a lot. Here are some thoughts on how I do it with a newbie. And I treat everyone like a newbie unless they give me very good reason to believe otherwise.
I hand them the gun by grasping the barrel/action from above, so the grip is available to them. I hold it in front of them, with the barrel pointed down-range, and let them get a comfortable grip. Before I let go, we have an explicit verbal communication where they confirm that they have it. And I stand ready to instantly take it back, if necessary.
Keep in mind that taking a gun in hand for the first time can be a very emotional experience for some folks. I've worked with women who have been battered/abused/raped/etc. The act of holding a gun can be a huge emotional step in someone empowering themselves to protect themselves. It can open emotional floodgates. So be prepared for them to shake, sob, collapse, etc. It doesn't happen often, but it does happen. Just take the gun back, support them, and pick up where you left off once they collect themselves. But be prepared.
Firing the first shot can have similar emotional impact. It's not hard to cope with this if you're prepared for the possibility. Just take control of the gun while they pull themselves together.
First, have them dry-fire. Use a snap cap if your gun can't be dry-fired without damage. During this stage, you're checking their grip, stance, finger outside trigger guard, and most important of all, their muzzle awareness. You're also getting them accustomed to the fact that you will be standing at their elbow whenever they're holding a gun. I usually start with a gun-shaped non-gun, but an empty gun can serve if you don't have non-guns available.
Second, load one round and have them fire that. Some people freak out after the gun goes off. Freaking out can range from dropping the gun, to throwing the gun, to emotional collapse, to pointing the gun at your navel while saying "That was so cool!!" with their finger on the trigger. I've seen all these things happen. If they're going to freak out, better they do it with an empty gun. So I load one round and let them fire that.
I stand at their elbow like an IPSC RO, but without the timer. My focus is on the muzzle first, and I stand ready to grab the gun if the muzzle looks like it might wander. If necessary, I grab the gun by putting my left hand over the top of the barrel/action. Their hands will be on the grip, so the top of the gun is available for you to grab. My left hand grabs the top of the gun, and that is nearly always enough. If I encounter resistance to my taking control of the gun, my right arm comes under their left arm, followed by my shoulder and I step between them and the gun, boxing them out while I control the gun with both hands. I've never had to use that maneuver, but I've practiced it.
After they've fired one round without incident, we move on to loading more rounds. We might do one round several times. At this stage, I trust them not to freak out. It takes a long time for me to trust someone's muzzle awareness. They get used to me at their elbow.
When I buy a gun, I ask permission to dry-fire and field strip it. If I were the seller, I'd include a field-stripping and lubrication lesson in the transaction. As a buyer, I expect to be able to return it within a few days if shooting it reveals functional problems. All of this presumes the buyer has demonstrated they have the proper paperwork to legally buy the gun. (LTC in Mass.)
FWIW,
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