Its just bad luck it happened so early on, damage like that is as likely to happened on your first ride or 100th and on a £400 or £4k.
I’d look at the cost cost effective way of sorting it. If the bike rode well fix it/or replace the arm and keep it.
If it didn’t decide if your going to sell it damaged or repair it then sell it.
Oh and sorry, sounds like its all a ballache.