I bet this has to do with the re-assessments. (Forgive me but should I say re-asses. What a fiasco that turned out to be in most counties?)
I'm not aware of any guidance in the IC, nor am I aware of any in RESPA.
My suggestion would be to have the borrower submit his request in writing stating the fact he is disputing the reassessment and that they will not hold you liable for any penalties they may incur should they lose the dispute. I would also strongly suggest to the borrower that they should at least pay last year's billing and only withhold the disputed portion.
A quick call to the Treasurer's office might also help.
As an after thought - unless this county was one of the ones that were late getting the reassessments out, I think, but don't hold me to it, the property owner only had 60 or 90 days to appeal the reassessment. A call to the Treasurer's office sounds more prudent now.
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The opinions expressed are mine and they are not to be taken as legal advice.