There have been court cases allowing exceptions for beneficial ownership issues, but that's not your question. You have to be legally liable to make the payment and actually make the payment to deduct the payment. If he owns the property but she makes the payments and they file MFS, then no one gets a deduction. That said, certainly they've agreed in the past. Do they put funds into a joint account to pay for joint living expenses? That would give them more flexibility to divide deductions they both qualify for after the fact when they see who would benefit. As long as one of them is not reporting a number higher than was reported on a 1098 in that one's SSN, then how could there be flags. If only one owns the property, then you get into nominee situations or you do raise red flags? Have you discussed the advantages and disadvantages of MFS vs. MFJ with them lately?