This is a MacOS system preference, Appearance->Show scroll bars. Automatic shows it if a mouse is connected or hides it if a trackpad is connected, "When scrolling" hides it unless you're scrolling, and "Always" will, well, always show it. You can't control whether it always shows on your website, but you can style it as mentioned in another answer. If you want to see the website like the other user, you'll want to set "Show scroll bars" to Always in your Appearance preferences.