Section 65 of the contract act says that "Obligation of person who has received advantage under void agreement, or contract that becomes void.—When an agreement is discovered to be void, or when a contract becomes void, any person who has received any advantage under such agreement or contract is bound to restore it, or to make compensation for it to the person from whom he received it. —When an agreement is discovered to be void, or when a contract becomes void, any person who has received any advantage under such agreement or contract is bound to restore it, or to make compensation for it to the person from whom he received it."
By the above it cannot be said that the lease of property from the society has been affected by the developments in the partnership firm. The society has leased out the property to firm and not to the partnership deed. If the contract i void between the partners, the partnership will be dissolved and new partnership can be formed in the name of the firm so there is nothing that is going to bother the society in this regard.
However, for the fraud played by partner B against the firm or A has to be dealt with in accordance with the law and rules meant for it on that aspect. That is a different subject. Partner A an even book partner B for his fraudulent acts including forging his signature, fabricating document for wrongful gains, cheating,and other issues involved in it. Besides, upon the resignation or retirement of a partner the partnership shall stand dissolved, therefore it is the fault of A to have continued the firm under partnership even after a partner has retired, so first introspect your own position before initiating any further action in this regard.