It happened to me once as well. My best friend, to the point where people in his town think we're brothers due to the amount of time we're together. I was going to head to the local gun shop and he wanted to come, no big deal jumped in my car and headed down. I was looking at the milsurps
"Hey Jay"
"Yeah?"
"Take a look at this one"
I put the K98 I was looking at back in the rack, turned around and was promptly staring down the barrel of a Mossberg shotgun. Proceeded to duck as he was laughing. We left the gun store about a minute later. I was pissed, didn't say a word to him, he knew he had done something wrong, kept trying to apologize, saying it wasn't loaded, but I just kept telling him to shut up. He got a little worried when I got on the Parkway, asking where we were going and the like. Simply told him to sit there, shut up and don't say anything. I stopped outside the office of my local range. One of the range officers there is a Vietnam veteran and openly talks about his experiences over there. I told my friend to sit, pulled the range officer to the side and explained what happened.
You see, there's two safety lessons the range officer gives, the standard, doing this because you have to know it, or the you just messed up big time speech. My friend got the messed up big time speech. Complete with stories of what a bullet can do to a person. We left there, no words were said in the car, he just kind of sat there looking out the window. We didn't talk for about two weeks. I went up to check on him and his two sisters. He was surprised to see me saying "After what I did you still want to hang around?"
He was more ashamed of himself for what he did once he realised exactly what could have gone wrong. He's one of the safest gun handlers I know now.