I would replace both, as matter of course. Likehood (IMO) is that the other spring won’t be far behind in terms of fatigue, so might as well do both.
Not all garages are like that though, they may believe that you don’t want to shell out for two new parts when ‘only’ one is needed.
Bust one spring (front) on the Saab 900 many years ago – local garage fitted a new pattern spring. When was next in for service in indy Saab garage down in Edinburgh, the guy replaced both, with Saab springs.
Recently, one of the back springs went – different local garage gave me the option, went for both (L+R) but went pattern springs – reckoned car is old enough that may not be needed to have original parts any more..