This is a repairable issue as its simply a splined rod that had become detached from one side of the seat chassis.
As has been said before, carefully remove the back from the seat, and look for a rod going from one side of the seat to the other. Put the seat backrest into the correct position, shove in the splined rod, with maybe some Locktite thread selant to hold it and bobs your uncle.......
Cheers,
Tony