Two options.
1) Have them tell you what the password was and then reset it.
2) Hopefully you got the original media, Snow Leopard, that came with the Macbook Pro, so take that hold down C during boot up, and use Disk Utility and erase everything and do a clean install and set whatever password you want.
My recommendation is for #2. You don't to have stuff left over from the previous owner, even if they are a friend..