Legal notice from a company previously worked.

My wife has joined in a pvt company in Feb2017. the company has asked her to sign bond which is on a paper with company seal which also the offer letter. the bond says that the company has hired her as software engineer trainee and she has to undergo training for a brief period of 6months. it also states that after completion of training she will be given a permanent employment. it is written in the document that during training she will be given 5000 rupees per month as salary and after that for 3months she will be paid 8000 rupees as salary and from fourth month after training she will be given 20,000 rupees as salary. but, the company has failed to give 20,000 rupees the fourth month after training and has given only 8000 rupees and moreover the company has started scheduling interviews with their clients for my wife with fake experience and apart from this they also had harasses her mentally. due to this she resigned with the company in dec 2017. and now in june 2019 on 17th june 2019 they have sent a legal notice asking to pay for the bond money. now what the action we need to do? actually the company has first disobeyed the agreement without paying her 20,000 the fourth month after her training.