Employer asking for money settlement from the employee post termination.

My brother was employed in India sales team for a Europe based B2B Travel company & was recently asked to leave the organization with immediate effect basis some allegations made on him. Since his side was not listened to by the directors & he was anyways not paid much due to COVID 19 situation he agreed to put down his papers by settling the necessary dues to the company & returning the company assets. But when he went to ask for Experience letter, the company directors delayed the same & later started accusing him of fraud in some files where his friends or families had travelled (showing that there was adjustment done in those files from some other files, which was actually done basis conversation with some of the directors only, but the conversations were not recorded). Now the directors are writing emails to claim the amount from my brother for all such files.
To this when he asked to gain access back to his machine / laptop so as to check all stated & related files; they denied the request for access to device but stated that they will share all necessary info over email (and some of the file dated back 3 years). Without any payment accessing files over the period of 3 years & providing justification did not make any sense & hence my brother denied the request.
Now the directors are sending mails stating "Rest assured that we will pursue this matter until every cent and centimes has been repaid". What should we do?
Note that the original agreement that my brother signed with the company has no such clause of payment to company or employee being answerable to the employer for any enquiries post termination.

Further to this, the directors of the employer are now reaching out directly to the Friends & Family members who travelled, claiming that it was identified in an audit that the payment is due from their end & they should settle it immediately. Seems that the intention is either (1) To extract the money from the travelers (which is unlikely as they have already paid their part & the company would not have allowed them to travel or given travel vouchers if that was not the case) or (2) The directors want the travelers to explicitly say that the money was handed over in cash to my brother & use that as a proof against him that he never submitted that money to accounts & without access to his mail-box my brother has no proof against them to prove otherwise. (Now the catch here is that cash was not supposed to be accounted for in India, it was supposed to be transferred / handed over to Europe branch & reported there by the Accounts basis confirmation from Directors; but several times directors had not reported cash immediately to adjust in other transactions)

Looking forward to some guidance please. Thanks in anticipation