I never use the handbrake on any car which is not in regular use, all to often they get stuck;
however that will not help your Father in his current predicament, so, maybe try:
1] If it's electrical, possibly some of the car modules 'gone to sleep' and some modules needs a little waking up;
on Audi's you can do that by turning on the indicators, putting the headlights on high beam and off a few times, using the wipers and similar.
2] I'm very skeptical that MB will, again, be like Audi, but on the Q5 there is a pin located behind the automatic shift, to get from P to N when needed,
I don't know of any such facility exists for the handbrake - either on VAG or MB cars - so trying to wake up all modules is the best I can offer.