Will it be considered a fraud if worked with law firm after losing postdoc job? I have an NIW approval with EAD

You can work with the law firm as you have NIW but you need to make sure that the work you handle should worthwhile to the United States.

Do you have work experience in the legal field?

You should work in National Interest then it is OK.

But if you were on OPT then working with a lawyer would not be considered as part of OPT training if the legal field is not your area of study.

I suggest you should consult an attorney first.