SNAFU

OP, your father is still listed as the Registered Keeper (RK) of the vehicle and has not updated the V5C address with the DVLA — which is a legal requirement. Failing to do so risks a fine of up to £1,000 under registration regulations.
In the meantime, all PCN correspondence is being sent to an address where he no longer resides. If a claim is issued and goes unanswered, your father could receive a County Court Judgment (CCJ) by default — even if he never sees the paperwork.
If you’re in contact with him, he needs to take responsibility and decide how he wishes to proceed. He has two initial options:
1. Update the V5C address with the DVLA so that any further correspondence regarding the vehicle reaches him at his current residence.
2. Transfer ownership of the vehicle to the actual keeper, if he no longer wishes to be associated with it or be held responsible for future issues.
3. (Optional) If he does not wish to deal with the PCN himself, he can formally name the driver and provide their full name and serviceable address to MET Parking. This discharges his liability under the Protection of Freedoms Act 2012, and the PCN will be reissued to the driver, who can then deal with it directly.
4. (Alternative to point 3) He may choose to appeal the PCN as the Registered Keeper, provided the deadlines set by MET Parking (or their debt recovery agent) have not expired. However, if the PCN is already at the debt collection or court claim stage, the right to appeal may no longer be available.
Only the RK can take any of the above actions. Until one of these is done — especially point 3 — no one else (including the driver) can deal with the PCN.
Point 3 (naming the driver) passes responsibility to someone else for the PCN itself, but does not resolve the issue of points 1 and 2, which still remain relevant for DVLA compliance and future liability.
If he chooses not to name the driver, point 4 allows him to retain control and potentially challenge the charge directly as Keeper.
Until one of these steps is taken, particularly 3 or 4, there is nothing the rest of you can do. The PCN remains his issue to resolve.