there's a way to find back the building, the compass is not the way , the compass is follow you to the place thats you spawned for the first time, like when you sleep in a bed and it broken, (can be changed by /setworldspawn)
if he build it so far from the spawnpoint, try use "MCedit"
its one of the best ways to make an large building and build things.
Can be downloaded from this link http://khroki.github.io/MCEdit-Unified/
after you downloaded, select your world and find it, its will be more easier.
when you found, go to the "Players" tab and select your minecraft username,
then click on the "camera view" and its will teleport him to you.
and just save and close.
go back to the game and the first thing you need do is /setwoldspawn
than you can use the compass to go to your builds without lost them again,
i sure thats i helped.
if you not understand what you was need to do, check some vedios on MCedit v1
for how to use.