There's not been any such thing as a Supersaver Return for quite a while. If you mean a Super Off-Peak Return, then the outbound leg may be used any time on Saturdays, Sundays, and bank holidays; on Mondays-Fridays it may be used for trains timed to depart between 1010 and 1501 or after 1900.
(Restriction code is YU, for reference.)
thanks island
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Is it the same time restrictions for the return portion?
If he wants to travel on an off-peak, all he'll have to pay is the difference in price, approx £13, either at the booking office, or on-board.
The difference in price should be the full difference, and as far as I am aware, this is reflected correctly in Avantix.
As Flamingo says, the price is the same on board as it is from a booking office.