Acid was poured in the male lover’s face in a staged attack, with the pair planning to say it had been transformed after surgery, police in the southern state of Telangana said.
The husband’s parents believed the acid attack story and paid hospital bills.
But the husband’s brother suspected a plot when he visited the hospital.
He made a complaint to police who carried out fingerprint tests and found the discrepancy.
A police complaint has been filed by the brother against the lovers, accusing them of murder.
The wife, Swati Reddy, has been arrested. Police told BBC Telugu they will also arrest the lover, Rajesh Ajjakolu, once he is treated for his facial burns and discharged from hospital.
The husband, Sudhakar Reddy, was allegedly killed on the night of 26 November and his body disposed of the next day. It has not yet been found.
Police say the woman has confessed that the pair killed the husband.
Rajesh Ajjakolu was admitted to hospital on 28 November, with Ms Reddy informing her husband’s relatives of the supposed acid attack.
Police said Mr Reddy’s parents believed that the man undergoing treatment was their son and they paid up to 500,000 rupees (£5,821; $7,758) in hospital bills.
The alleged plot was not uncovered until 9 December, when Mr Reddy’s brother visited the hospital and lodged a police complaint after suspecting the man to be an impostor. (Courtesy BBC)