That'll be the problem. Linux is highly interdependent and everything must be upgraded together to stay in step. This should be done fortnightly for pclos to reduce the chance of problems.
If it is several months since the last upgrade it might be too late to upgrade. In particular you will need to take heed of http://www.pclinuxos.com/forum/index.php/topic,94949.0.html
to upgrade the package management system before upgrading the rest of the system. A reboot will be needed.
There is no guarantee it will work after such a long time. If it doesn't you will need to back up and re-install from the latest ISO.
As it is a friend's machine, you will either need to visit regularly in order to upgrade, or set it up to upgrade automatically at a time you know it will never be switched off until the process has had plenty of time to complete. (There is not currently a lock on the process which will prevent the machine being shut down during upgrading and that would be almost certain to destroy the OS.)
Alternatively, never upgrade at all and just re-install periodically to stay reasonably current. However, that has security implications, especially if financial transactions are involved, as there will be no security updates.