I dont see why they wouldnt return, thats a pretty big fine. I have been in the situation before where the groundstaff forgot to give the pax a form saying 'I know i havent a visa but ill chance it', and in that case it is the airline that pays up not the pax. Need to weigh up cost of return vs cost of fine, and also the date.
If they were only going to italy they may not have needed to dump fuel (may have taken off with less than MLW ??).