My 10 year old son has been opening knives and utility blades for years. If an adult can't open and close a knife, I'd question if they should even use one.
And I imagine you took the time to teach your son knife safety, and proper respect for knives, before you ever handed him one, or let him use one on his own.
I have no way of knowing if the OP's friend has ever received such instruction, or if the OP intends to offer such instruction, or if he's just going to give his friend a knife that he has zero experience with. The OP only says that his friend has "seen" his knives.
I'm 52 years old, I was taught knife safety and proper respect for knives as a child. I started with slip joints, and after demonstrating safe handling of slip joints for several years my parents bought me a back lock. I'm glad I started with slip joints, because it taught me to use every folder like a slip joint. As a result I never have to worry about a locking folder accidentally coming unlocked and cutting my fingers.
For a person I don't know, who might have zero experience with folding knives, much less one-handed folding knives, I would recommend the same- start with a slip joint, learn to use it safely, learn to treat every folder as a slip joint, and then move on to locking knives.
I don't believe that any person is born knowing how to properly and safely open and close all the various types of folding knives. We all had to learn. Not knowing how shouldn't exclude someone from carrying or using a knife, it just means that they should learn, if they want to. And if they have no interest in learning knife safety, or proper respect for knives, then I would say they definitely SHOULDN'T be using knives.
I didn't touch a liner lock until I was 32 years old, and had zero experience with them, so I asked the owner of the knife store to show me the proper way to open and close them. I remember I kept trying to push the thumbstud straight out, which didn't work. He taught me to push it forward, and that's how I learned. I'm sure I could have eventually figured it out for myself, but I always appreciate learning from those more knowledgeable than myself, and I'm never ashamed to ask them to teach me.
Twenty years of using one-hand folders later, and I've never cut myself opening or closing one. And I'm still thankful the old guy at the knife shop took the time to teach a 32 year old guy how to do it, rather than telling me I shouldn't be using a knife if I can't figure out how to open and close one.