It is owner occupier mortgage interest relief - she is not an owner occupier of her house so should not be claiming the relief. She can claim such relief on the family home mortgage though. She needs to sort this out ASAP and repay any mistkenly claimed relief. There are lots of other potential implications here - e.g. she should be registered with the PRTB, should be declaring rental income, she can offset interest on the mortgage against rental income but only if registered with the PRTB, if she rented the property out within 5 years (prior to November 2007) or 2 years (since November 2007) of purchase as an owner occupier then she could be liable for a clawback of stamp duty on the original purchase. She probably needs to get independent, professional tax advice to figure out any anomalies in her situation other than the mortgage interest relief one and then rectify them.