Hi,
First and foremost, one shouldn't resign without giving notice period or settle in lieu of notice; as per your contract or letter of appointment clause. If he had a "Probation Period", he has every right to resign without notice period when on probation. Check the Contract.
Secondly, one have to understand that you are not slave to any company and nor any company force you to work let alone call you and threaten you, whatever is the reason. Company cannot put on stress by such act which will go against the company in court of law.
Thirdly, it is very important to understand what position your friend was working. If it is a sensitive work then its very important that your friend meets the company and settles the issues. Take it from, No company in India can do any harm to him nor stop him from working for any other company for his betterment and as per his wish. So, just ignore their call its just one of their cheap tactics.
When I say, please note that it is equally important that an employee must also be fair and not act as if he is without any onus of responsibility with his Employer and the Job he does. Employee is backbone of any company and such childish act disturbs the very business on which they survive and many other's livelihood.
Regarding Blacklist, companies cannot just Black list any employee because he did not serve the notice period. So relax and ask you friend to talk to them and also tell this Company not to terrorize him with such threatening calls or else you would have to "FILE An FIR" against them and also sent a legal notice Solicitor to put in stress, tension and mentally disturbing your peace with alimony.
Good luck...
Ukmitra