We are a group of 4, tickets booked together, now one is ill and can't travel with us. Will it be a problem when we will use the ticket to the nighttrain where we have a cabin, and the greek ferry where we also have a cabin? We can't change the reservation, we paid for it already, and are not trying to get a refund. 

But I seem to remember I read somewhere, that all names on the ticket needs to check in.. but I simply can't find the answer anywhere. And the companies didn't help. Perhaps someone tried something similar?