You may have a tough time convincing this friend of yours that they do have personalities. I know someone who prides herself on being very logical and science-minded. She thinks in a very black and white way and needs to have something definitively proven or disproven to her via intense scrutiny under the scientific method, otherwise she will absolutely not budge on her views.
Since "personality" isn't really something that can be proven out in scientific terms, she of course refuses to believe that reptiles can have different personalities. She is also still of the firm belief that snakes are purely "instinct-based" as opposed to mammals. She's of the idea that dogs and cats can "love" people but that reptiles are physically incapable, due to brain structure. Me? I'm an agnostic so I will just say "I don't know" on the love issuewhich means I'm not saying that they can, but also not saying that they cannot. How could we possibly presume to know something like that? But other people are more certain, even to the point that they will ignore people's experiences and dismiss them as useless anecdotal evidence.
My view is that... Of course snakes have different personalities. They act differently under the same circumstances. They're not rocks.