get a second opinion at the least.
Is it leaking fluid from the calipers? Then they need to be new/rebuilt. But they don't necessarily need to be replaced in pairs unless both are leaking.
Not leaking? Did they take the calipers off and try to compress the piston and find that it does not compress? (sometimes there are ways to tell if the caliper piston won't compress without removing the caliper)If the piston will compress smoothly and easily, it is OK.
Many dealers and other companies also will just over sell you.
However, if one side of the axle is leaking, replace both axle seals (both sides), due to the need to drain the diff fluid. The seals are cheap.