I wonder how much financial gain the guard made for selling the ticket. Was this a genuine mistake (in which case an £80 fine may be appropriate), or was this a deliberate fraudulent attempt to make money?
Fraud: noun: deceit, trickery, sharp practice, or breach of confidence, perpetrated for profit or to gain some unfair or dishonest advantage.
I'm sure it's just a genuine mistake from someone that doesn't know his job - I'm aware of at least 2 people in charge of trains (on a different TOC) who have no knowledge of the national rail conditions of carriage, let alone have read it.
However even in this case, the ticket clearly says to check
nationalrail.co.uk/2V for restrictions. The guard has ignored this, as if he had checked those restrictions he'd have seen
Return Travel: Not valid for trains scheduled to depart after 04:29 and before 09:30.
Overnight Break of Journey - restarting journey from an intermediate station: return restrictions as above apply from the intermediate station, Monday-Friday.
(This is assuming, as the OP states, that "All travel would be between 10am and 7pm each day.")
It's all very well saying "write in and
request a refund when they're the ones in breach of contract". This "pay when challenged" mentality that TOCs have is simply not on, and why there should be real penalties for them, upto and including jail time.
N.B I don't really believe the guard should be fined £80, or go to court, however I don't believe that Northern's customers who make far more honest mistakes should be fined £80 or go to court either. In both directions, an honest mistake should be dealt with by charging/refunding the appropiate fare, with an apology. Any genuine deliberate issues (like fare evasion, or the schenanegans going on at Paddington and Guildford) should be dealt with in the courts.