Yeah, even though that 10% is net tax 8.5% (vs. the full 6%) it seems like one could refinance it down in future. Still, I don't think most payment terms are flexible enough to make them an indefinite loan (go interest-only).
Depending upon their tax situation, the loan rate may be effectively a little lower due to the tax deduction too, though. And, tbh, 6% is a really high mortgage now. Rates below 3% are common.
