I'll throw out a few ideas:

1. You said AT her home.  Are the panels connected to her home meter, and therefore powering her home?
2. The taxpayer has no tax, so the other tax preparer meant that there would be no current year benefit due to it being a non-refundable credit.
3. The other tax preparer was mixing it up with the $500 credit, and the taxpayer had already maxed out the $500 credit.
4. The other preparer had some other bit of information that we don't know (although I can't really think of what that could be).
5. The other tax preparer was wrong.