You are not doing anybody any favors by installing just one strut, spring, or bushing.
While the bill might be half the price, they could be paying more than the full price down the road.
This car had a broken right rear spring. The owner replaced only the spring and strut in that corner. This made the handling erratic and the headlight patterns were not aligned with the road.
This pickup needed a new right front lower control arm because the bushing was torn and causing noise. The owner installed just one new control arm.
Now the left front is making noise, and the inside edge of the tire is worn to the steel belts.
In both cases, doing half the job compromised the safety of their vehicle. This is why some chassis parts should always be replaced in pairs.
This video is sponsored by TRW.