Issuance of Releiving letter

Hi , My friend has been working in the private organisation for last 2 years.During the last 4 months of her jib her immediate manager resigned and no one was being hired for the position. She has to work day andd night to handle all the activities. Doing all this for 4 months her health got deteriorated and on doctor further advice of bed rest for one month decided to focus on her health. Fed up with the company attitude and many request to fill the vacany , She has tendered her resignation over email to HR citing the medical reasons and asked for immediate relieving. Also she kept chasing the company to take over the laptop and other data but they didn't listen. One day suddenly they asked for laptop and data and got to know they hired consultant for the job. we have submitted the all the medical records on being asked by the company. Now the company is threatening to not issue the relieving letter. The notice period is for 45 days and no buy out policy is specifically written in contract. Can i just send the cheque for buy out period and claim the relieving letter as Holding an employee on medical ground and disabling her to get another job is against the one fundamental right.