" Two wrongs don't make a right " is a proverb used to rebuke or renounce wrongful conduct as a response to another's transgression. Therefore, no use in delving deep into the legal aspects of the situation just because of the absence of appointment orders or stipulation of a specific notice condition by the employer under whose service the employee is supposed to be still logically in.
As a recruitment consultant, you are bound to recommend a candidate based only on the conditions stipulated by the prospective employer who is your principal. If the candidate is also very particular about getting a berth in your principal's organization, ask him to pay the notice amount and obtain a formal relieving orders from the previous employer.
i have in my decades of recruitment experience, never come across a candidate doing this
however looking back at the whole interaction with the candidate and his approach.. his appointment has been withdrawn