First and foremost, one shouldn't resign without giving a notice period or settling 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 a notice period when on probation. Check the Contract.
Secondly, one has to understand that you are not a slave to any company, and no company can force you to work, let alone call you and threaten you, whatever the reason may be. A company cannot put stress on you by such acts, which will go against the company in a court of law.
Thirdly, it is very important to understand the position your friend was working in. If it is a sensitive role, then it is crucial that your friend meets with the company to settle the issues. Remember, no company in India can harm him or prevent him from working for another company for his betterment and as per his wish. So, just ignore their calls; it's 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. An employee is the backbone of any company, and such childish acts disturb the very business on which they survive and many others' livelihoods.
Regarding a blacklist, companies cannot just blacklist any employee because he did not serve the notice period. So, relax and ask your 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 send a legal notice. Solicitors can put stress, tension, and mentally disturb your peace with alimony.
Good luck...
Regards,
Ukmitra