Pictures would help.

Has water got into the plasterboard and right through the stud wall? If so it might have to be taken off to allow it all to dry out. Could be a right mess inside the wall if it's been leaking for years.

Ideally in or near a shower you would use a waterproof board - perhaps Hardi backer board under tiles. If the wall is part brick, part timber it might be best to clad both with hardibacker board (eg across the join) so that any future movement where the two meet doesn't crack tiles.