The first think I check on PS4's I get like this is the ribbon cable between the motherboard and optical drive. The little ears on the cable easily break off when pulling the cable out, then when you go to plug it back in it can't plug all the way in. If that's the case then you'll need to either fish the little ears out of the connectors and replace the cable or replace the entire connector and cable.
If that doesn't fix it unfortunately there is no other easy fix. They can always use it to download games.
I hope this helps.