My account is my friends account...??

I just got this app, and so did my friend. I was running into Error 10010 on my phone so I asked him to sign into his account from my phone. It worked, no error, so he logged out and I signed into my account, which then was just HIS account with MY screen name on it?? Now neither of us can be on at the same time, or we experience the "You signed into this account on another device" message, and are logged out. Help?????