Tds on interest income from nro accunt

One of my relatives is an NRI, residing in OMAN and having NRO fixed deposit account with Indian Bank. The Bank is deducting tax @30% instead of 10% as per DTAA with OMAN.While contacting with the Bank, the Bank official has told to produce Tax Residency certificate (TRC) for getting benefits of DTAA. He has contacted the authorities of Oman, they have denied issuing such TRC. can you please advise what my reltive has to do for getting the benefits of DTAA.