I'm also not convinced that it shouldn't have been a rental property either. 🙂  See if you can figure out what a comparable place would have rented for.  I'm assuming "paid the mortgage" probably also included escrow which would have covered taxes and insurance.  In some areas that might be comparable.  Have the client consult with a realtor (i.e. the one who just made commission on selling their house).  I think it's fairly easy for them to come up with some comps.