score:8
Eurostar conditions of carriage do not mention this, and this thread on flyertalk (dating from 2010) seems to argue that you can use just the return journey without taking the outward train. According to several users, return tickets are treated like two singles and are therefore independent.
Upvote:3
On the eurostar FAQ:
If the traveller doesn't use their outbound ticket, can they use the return ticket?
Yes as long as you leave the outbound journey alone and in a Live status in your GDS, the passenger can use the inbound ticket.