Legal right for Employee

No. It is not valid even if he has signed such clause during employment. Post resignation there is no such question

The facts, as posted, lacks clarity.

Whether your Husband signed any service agreement at the time of joining. It is a normal practice for any commercial organisation to ensure that a person employed with them does not utilise the expertise/contacts after resignation and become a competitor. If you go to Court, you may get a judgement in your faour or against it may be after 5 to 6 years. Now what the employer is asking is that your husband should not contact the existing contacts of the NBFC,. Just give a delcaration that no such contact shall be made say for a year or two. It will solve the problem.