I have my friends passwords and I need help.

So basically a friend had gifted me a MacBook, and he had factory reset it, so I logged In but there was one app I wanted on it so he logged back in downloaded it logged out of the MacBook then I logged back in, a couple days later when I was logging into a website on my phone, his passwords popped up and it confused me. Just wanted to know if you guys know about an issue like this and how can I resolve it.