She won't start!

[Rookie]

Hi, I have a bit of a problem. Lent my landy to my dad who's 400km in the bush. He called and said the landy won't start when he turns the key. I was sure he simply pressed the lock button inside the cab before he started the car and therefore the problem. But when I said to press the unlock button on the key and then try the ignition --- no go. Called land rover dealer and talked to a middleman who talked to a technician and they said try what I told my dad already or unplug the battery (key? or Vehicle?) and it may restart.
Does anyone have any advice on how to restart or reset the arming device on my vehicle so I can get my dad on the road?

Thanks