After database import, username is invalid

A friend set up a WordPress site on his local drive with MAMP. After I imported the theme, images & database to my own local host, it shows up OK, but when I go to log in, it say the user name is invalid. I checked the config file and user privileges, and everything looks OK. The same user/password combo worked before I imported the database. Could it be something in the wp_users or wp_metadata tables that were imported with the database? They don’t list my user name when I query them. Is there some step I need to do before importing a database so this doesn’t happen? Any ideas how to fix this? Thanks!