• What actions would an employer take for not serving notice period

My wife submitted her resume on 20th April and have requested both verbally and on emails to the team to give a feedback on the resignation letter. She was not paid for the month of April and as the employer mentioned verbally that she would need to serve the notice period free of cost for 2 months. The appointment letter signed, mentions 30 days notice period which would be a paid notice period. The employer did not pay her for the month of April and she decided to leave the organization on 4th of May serving half of her notice period. 
Everything mentioned above is also documented in an email copy other than the employer accepting the fact that they will not pay her. The bank statement shows that they did not pay her for the month of April

Reason for leaving - Mental stress at work, abusive languages being used while at work but all verbal conversations, Late night work and working at the office after office hours continued a lot. Due to the negative atmosphere, she decided to take a break and look out for a different firm or start on her own.

Current Status - She is working with a different organization and have joined in the month of June. She has kept the status clear with the existing organization for what all things have happened and why is she not carrying her former employer relieving letter. The current organization liked her skills and creativity and did not mind about the documents. They also checked with the reputation of the previous company and found similar complaints by other employees on Glassdoor.

After few months as the previous organization came to know about her current employment status, they have sent an email to the current HR team from the new organization asking about the dues she needs to pay, Client IP she has and mentioned that she has absconded.

Previous Organization Background: There are 2 co-founders. Whenever they use abusive languages to the team with women employees, the lady co-founder yells at the team, some have even recorded few conversations. The male co-founder always avoided to talk to female employees on such matter and redirected it to the HR head or Lady co-founder.

Question:
She did not get her 1 month salary, is she still eligible to pay any dues?
She worked with the client on their own systems and have made sure everything was transferred and shared with the team, does not have any IP for any of the clients as she just have submitted everything to the organization few days after resignation, what are they trying to get the employee involved in here for?
Similar situations have been made to other employees, due to which some were not able to join in new organizations or are still unable to work openly. How can you stop them for fooling others? Few of the others who have got in touch with my wife post her resignation did not receive their salary either.