Forcefully extending the notice period of an employee

One of my friends is working in the UAE for almost 5 years. He got a new job and resigned from his current job with a one-month notice period as per the contract.

He was also required by the new employer to join him within one month.

But his current employer is threatening him to spend at least 3 months with him as notice period or he will make any case against him to stop him joining the new company.

Can you please help in the light of the UAE labor law?