Last month salary for employee left with our serving notice

Demand to supply ( in writing under proper acknowledgment of course) the acknowledgment and acceptance of resignation, correct  FnF statement showing computation of earned wages till LWD/bonus/leave encashment, adjustment of notice pay etc, Form 16 as per correct FnF statement,service certificate, relieving letter,PF a/c number and slips etc

The employee was in which state?

What was her designation and nature of duties, length of service?

As you said, you wife resigned from the job. Was her resignation accepted? If her resignation was accepted with immediate effect then it means she was not required to service the notice period. 

If not, is the notice period not served and the amount of salary not paid, equal (e.g. you did not serve notice period for 1 month and salary of 1 month was not paid)

Under the provisions of labour law, salary cannot be deducted on any invalid ground (there is no provisions of deduction of salary through notice pay, though many employers do that). In this case, depending on the nature of employment as well as nature of the industry, one can claim his outstanding salary from the management. 

The querist has not replied to the points raised.

If the said resignation was Notice of resignation/or relevant clauses of appointment letter was quoted/or LWD was mentioned......................................then if employer has accepted before expiry of notice/LWD then employer shall tender notice pay and employer can reject the acceptance and afirm to serve notice period.................. 

However you may show the appointment letter, all rules/policies quoted in it, notice/resignation and its acceptance to an able counsel specializing in labor-service matters for a considered opinion.