My mate bought me a ticket with his railcard still on and the ticket inspector issued a fine to me. I couldn't reas9n with him at all.
Sent an appeal and it got rejected
Any advice on what I should do next?
Probably (I'm sorry to say) that the best advice is that you should pay it promptly before the £50 discount deadline for prompt payment expires.
The reason I'm giving this advice is because basically for an Appeal to be successful you have to state why the penalty fare should not have been issued within the scope of the Penalty fare rules (ie what aspect of those rules were broken when the Penalty Fare was issued to you) - the Appeal process is not going to be successful by appealing to their better nature in such terms as arguing that your mate made an honest mistake and it was one off, and the Penalty Fare is unfair, sadly.
There might be some loopholes that Penalty fare experts can advise you about (eg the penalty fare issued was wring in that the paperwork you were given was incorrectly filled out for example). But that would be different from trying to reason with them.