Tell him to download the new duplicator. The old duplicator was coded to only work on a person's bricks if you have full trust with them, and, obviously, you always have full trust with yourself. But in V12 a third trust level was added that you can't give to other people. Now instead of having full trust with yourself, you have level 3 trust with yourself, and the old duplicator will only work with full trust. The new duplicator, however, is coded to work with full trust or anything above it (I believe).
Oh yes, and what Regulith said.
