As far as I know, the SNCF site (tgv-europe.com) gives T&Cs for SNCF tickets. Eurostar has its own set of conditions, but even these aren't especially clear. The
CoC state that:
12. You may not transfer your ticket to anyone else
Tickets already partly used, or made out in a passenger's name, are non-transferable and may only be used by the person for whom the ticket has been purchased. Tickets may not be offered for sale except by Eurostar International Limited or its authorised agents. If a ticket is resold or transferred for profit or other commercial gain, it will become invalid and the holder may be refused access to the Eurostar train.
There's a little ambiguity in the phrase
the person for whom the ticket has been purchased. However, in the
FAQs, this is spelt out more clearly:
All Eurostar tickets are non-transferable
Please refer to our conditions of carriage point 12 which can be seen on our web site at the bottom of each page.
Tickets are only valid for the person whose name appears printed on the ticket.
Please note: If you have made a minor spelling mistake to the names on your booking this does not generally cause problems when checking in for your journey.
However, personal experience, and that of many others (there are sites dedicated to reselling French train tickets, and these include Eurostar tickets) would seem to indicate that you can get away with it, even with manual ticket inspections (don't forget you normally just put your ticket into a barrier, and immigration care about your passport, not your transport). But I wouldn't like to say how much you'd want to count on that.