Your wife and her partner have 'joint and several' liability to their creditors. That means that anyone who is owed money can jointly sue both of them or simply 'go for the easiest target' on her own (which appears to be your wife).
So your wife might well be forced to pay out because of her partner's failures. If so, all she can (under civil law) is to sue her partner to get back the money. So she might need this link:
http://www.hmcourts-s...bout/claims/index.htm
In practice your wife's best option might be to go to the police herself and then to write to the caravan owners, explaining the situation and assuring them that she will seek to get her partner to pay up. Doing so (and thus showing that she's on the side of the caravan owners) might reduce the chances of her facing aggressive legal action from them.
Chris