Locked phone with Android Device Manager and can't remember the password I lost my phone and used Android Device Manager to lock it. However, it turns out my boyfriend was last logged into my Gmail, so his phone is the one that was locked and now I can't remember the password!

How can we unlock his phone?
Only solution,Hard reset your phone and do a reset factory settings in recovery mode. BTW which phone model you are using, I guess it must be Samsung :)

updated

One more way, if you know your phone is rooted and ADB on..
You have to delete all your key files from system folder. Try this.
Connect you device with usb and run following command. If you are not able to do so, i am afraid. You need a full wipe.

adb devices
adb shell
cd data/system
su
rm *.key


Even if shows lock screen after reboot, try a easy random pattern/password.