This route may be of interest, had a look for a chum, it costs about 80 pounds return.
Portsmouth to Ouistream/Caen, foot passenger. Caen to Le Mans by train direct, or via Paris depending on time of arrival. obviously, just reverse out for the way back.
The rub is getting a train out of Le Mans on Sunday, its normally the TGV's that get fully booked. Or maybe hitch a lift back in a car to a ferry port. Getting on a ship to blighty as afoot passenger shouldn't be a problem.
The route is quite quick, with a bit of planning it should be a goer.
http://www.raileurope.co.uk/use this web site to do the rail links, fares from the channel ports start about 8 euros single from caen to le mans, if he's got a student discount card them maybe cheaper.