A €1 coin weighs 7.5g. The 3,700 remaining coins in their room (assuming they were €1 coins) would thus weigh 27.75kg. If this was split between the luggage of two persons, it would be quite easy to carry the coins. I am not sure about the customs side of it, but I can’t imagine there’s any reason to stop a bag with - let’s say - 10kg of coins inside. As for not exchanging them at home: many money changers only take notes and don’t want to handle coins. And of course it’s China, so people don’t want to take the money to a bank because there might be taxation issues (I’m sure the scrap merchants aren’t declaring their Euro finds).
Nice story on which to speculate… 
