The 14-year-old was undergoing treatment at the Government Rajaji Hospital.
The girl, studying in the ninth standard, was set ablaze when she was returning home on February 16.
Police arrested Balamurugan who had been reportedly pestering her to marry him.
The girl's parents had lodged a complaint against him with police.
Angry over this, Balamurugan waylaid the girl, doused her with petrol and set her ablaze, police said.
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