A few things going on there...
Presumably he put some tension on the cable or was somehow otherwise able to get to the latch.
Impressive given that the cable runs inside the engine bay and the latch is obscured by the cooling pack... Certainly, unclipping the bottom of the bumper or removing the engine tray is the only way you could expect to get an arm anywhere near either
You've had problems with the locks before. Mechanical locks need regular use to maintain reliability. That said without power, turning the key doesn't always pop the lock button, so it may actually have unlocked it. Also, the key needs turning with alot more force than might seem reasonable... (I have actually snapped a key doing this).
And, if your battery went flat in the time it took to go shopping, then you might need to revisit the charging system or consider alterations to how you use the car...
By this I mean that the battery can't have been fully charged to begin with, so either it's weak or the alternator isn't charging at 100% or the types of journeys your doing aren't enough to recharge the battery each time. None of which is helped by the dark evenings and cold wet weather.
Take the lock barrel out, clean and suitably lubricate it and relubricate the latch mechanism whilst you're in there, and carry out Kevin's starting/charging test and address any shortcomings you find.