Three main elements make a contract legally binding:
In my opinion, the consideration element is missing. The candidate must have reported to work to consummate the employment relationship(in this case consideration). But no! He never showed up. Company can let it go and call another candidate who appeared for the interview.
Offer letter and Appointment order are two different documents. Signing an appointment order or contract before joining the organization is very uncommon.
If the candidate and employer sign a set of terms and the content of the appointment order stipulates explicit terms for consequences upon not joining, then there is a scope for contract violation when the employee shows no interest in joining.
So, if this is just a formal appointment order the scope for a legal fight is too narrow.