They should have done their research a bit better. Greece is well known for having no sense of humour when it comes to taking photos of military bases without permission. As happened to some British and Dutch plane spotters who were actually convicted of spying some years ago.
Many countries are hyper sensitive about taking photos of military installations, unsurprisingly. Even here in Ireland it's illegal to take photos of military barracks even if some of them have been in place for two hundred years.

Not that anyone has ever been arrested for that here.
If they're lucky they'll get away with a suspended sentence. But they could do time.