I'm surprised no-one else has picked up on the areas highlighted above, but
a) OP's husband travelled to London Friday 18th March
b) OP's husband travelled back to Maidenhead Sunday 20th March
c) At some point, OP was made aware by her husband, that he had borrowed her season ticket
Surely, if c) occurred before the journey b) was undertaken - perhaps when husband and wife called / texted one another on Saturday - then the TOC would be far more likely to prosecute ? Because IF that is what happened, the owner of the season ticket should have taken steps to advise her husband to buy a ticket for his journey from London to Maidenhead. Not persuading him to do so could be construed as her being complicit in the misuse of her season ticket, as she could then not use the defence that she did not know it was about to be misused (for the return journey) :?
But other than the power of persuasion, as she was out of the country she had no effective means of preventing its misuse. Basically, the husband just did something stupid (twice). (Unless she actively left the ticket at home and encouraged him to use it)