It was about £40 for a anytime single I checked at the machine and at the ticket desks and neither had the option for an off peak single the train I got was at 14:16 (the aircon on the train didn’t work for some of the carriages but they did declassify first class since the aircon worked for those )
The cheapest anytime single from London to Birmingham is £58.70, so if you paid "about £40" you did not have an anytime single.
The cheapest walk-up fare for a journey from London to Birmingham on Avanti, which is the only operator on that route with first class, is an off-peak single for £46. The same fare applies from London to Sutton Coldfield, but if you for some reason asked for two separate tickets then that is what you would be sold. Avanti was the only operator running direct trains from Euston to Birmingham yesterday, so if you specified a direct service that would also have been taken into account.
I was told I could only buy an off peak ticket if it was a return journey same response I got at new street from avanti staff when I asked for an off peak single from new street to Sutton Coldfield
You have been correctly advised that this route does not have off-peak single tickets, only anytime and evening, and that off peak day returns are available. Fares on this route are set by TFWM so if you have an issue with that I would suggest contacting them.
Based on the information available so far I'm unable to conclude that any mis-sale occurred. There may have been a misunderstanding by the passenger, or potentially an unclear communication by ticketing staff, but it does not appear to me that there has been anything more. Denouncing staff as "spouting utter hogwash" seems an overreaction. If additional information comes to light, such as the precise price the passenger paid and the precise tickets they were issued, I reserve the right to revise this view.