The victim, Sahroon Datia, is undergoing treatment at GTB Hospital. Her partner Rohit Kapoor, 31, is a small-time property dealer and his condition is stated to be stable.
A case of attempt to murder was registered against Kapoor and he will be arrested after he is discharged, said police.
Datia is from Bihar and is separated from her husband, while Kapoor, whose family lives in Laxmi Nagar area, is also separated from his wife. The two had been living together for the past three years in a rented room behind V3S shopping mall.
On Wednesday evening, Datia had reportedly asked Kapoor for some money to visit her hometown in Bihar to file a case against her husband. "Kapoor refused to give her money as he feared that she will not return. An argument broke out between them after which Kapoor left the room. He returned with petrol, poured it on her and set her on fire," said a police officer.