Employment Agreement

One of my friend resigned from his job after working 3 years in a company.He mentioned orally due to his personal reasons he is resigning but in In his resignation letter didnt mention the reason but he mentioned as per company policy 2 months is the notice period, he want early relieve by serving 15 days notice. He received an acceptance letter from HR. His manager sent an email to management stating he have medical issues and required a complete bed rest and he offered my friend 3-6 months work from home and my friend rejected it but no where it is communicated to him by his manager. After completion of his notice period of 15 days he was relieved stating the certificate of service letter will be issued after one week. Immediately after this he got an offer and joined in a new company knowing this his previous employer sent a legal notice stating fraud, misrepresentation and dishonesty and misusing his health condition and informing him to pay two months salary to the company otherwise they will take legal action. Can anyone please suggest what action he can take now.