score:3
While the other answer is 100% correct you need to consider just tossing that ticket and buying a new one. Very broadly speaking, within Europe, you rarely need to pay 250 EUR per person even last minute. A quick look shows me tickets on Air Moldova and Vueling both for 205 and 208 EUR tomorrow, to save a little money but it's possible more search could find even lower β especially if their visa and circumstances allow staying for 1-2 days more.
Upvote:5
You are lucky that the return flights are usable at all.
Often, when you don't fly one leg, all remaining legs of the ticket are forfeited and lost.
The visa issue is exclusively the travellers responsibility, and no airline covers that risk for you. The airline let them board for the first leg, which they probably shouldn't have, but that would have been only the airline's issue if the visa's couldn't be cleared, and they had to fly back immediately (which was not the case).
So the airline allowing you to change the flights back for a relatively small amount, instead of asking you to buy completely new one-way tickets is already accomodating. Take it, and be thankful.