As yorkie has said, the correct thing for the guard to do is to issue an excess for the ticket. The relevant part of the Conditions of Carriage is as follows:
12. Restrictions on when you can travel
Restrictions apply to the use of some tickets (including those bought with a Railcard) such as the dates, days, and times when you can use them, and the trains in which they can be used. These restrictions are set out in the notices and other publications of the Train Companies whose trains you are entitled to use. If a restriction applies and the ticket you are using is not valid for the train you are travelling in, then:
(a) you will be liable to pay an excess fare (the difference between the price
paid for the ticket you hold and the price of the lowest priced ticket available for immediate travel that would have entitled you to travel in that
train for the journey shown on the ticket); or
(b) in the case of some types of discounted tickets (as indicated in the notices and publications) the relevant parts of Condition 2 or 4 will apply.
(b) generally applies to Advance fares only, so (a) applies and the guard should charge the excess: this should be for Euston - Wigan (i.e. the passenger cannot excess it shorter). This would be £235.00 minus £66.40 equals £168.60, so actually the SOS would be cheaper at £117.50, and a half-decent guard would give the passenger the choice (and you could just buy the SOS to Crewe at £95 instead). (Prices from NFM03.)