It is possible the software also saved a cookie on the PC to prevent being able to pass the login information around to others to use the same on-line course without paying for it. It is also possible that there is some tiny difference in the user ID or password that is not obvious. To rule that out I would copy/paste the user ID and password to an e-mail and either send it to yourself (herself) and copy/paste that on the tablet or the Linux machine.