He's just going to clone the key you have. Meaning, the truck will see it just the same as it does the key you have now. If for some reason the truck rejects that key at some point, neither key will work. The lost original key you don't have will still be valid as well with a cloning, meaning the original owner of the truck can still access the truck and drive it away if they have that original key.
The only way to do it "right" is to use Forscan or to have the dealer do it. You'll have two uniquely programmed keys and the old lost one will be wiped. A cloned key will work, but it's not the same by any means.