Install mobile spy and read text messages on the control panel?
Did you forget it or you want to steal other's password?
Install a keylogger on your computer, but you may tell them
If you are looking for a keylogger software to record password, I suggest you to use micro keylogger.
It is illegal to hack one's password, but you can ask him directly, maybe this can help you.
Aobo Keylogger is your best friend.