Did that one client actually receive 100% of the income and pay 100% of the expenses?

If not, both taxpayers may have 10 years of incorrect tax returns.   Even if the one person did 100%, it could depend on how the title of the property was held, as well as local state law.

Did both people actually purchase/pay for the house 50/50?  Did they actually receive 50/50 of the proceeds of the sale?