Any challenge wouldn’t be against the club’s T&Cs regarding sharing/passing on/lending your season ticket but rather against a duly enacted law of the land that specifically states that to do so is a criminal offence.
Good luck with that.
You’re missing the bit that makes it illegal to pass on a football ticket without the permission of the club.
The club never gave permission in the past to share a season ticket.
Paragraphs 3.1 and 3.3 of the season ticket terms and conditions.
https://tickets.brightonandhovealbion.com/screenloader.aspx?type=include&page=usercontent/documents/html/tandc.html#sttandcs2122