A. Is there any partnership deed between A and B ? Whether it is registered or not ? Is property purchased in the name of partnership firm or not?
B. As per your statement, A and B has got property by register paper. Hence, A and B has joint ownership over the property. A and B has 50% share accordingly. Therefore, B cannot do anything without A consent, i.e, Sale, mortgage for loan etc.,
C. If the property is registered, A can apply certified copies of the paper before the local sub register office to know the terms of legalalities. And A can approach the court for an injunction to stop the work if B starts the construction. Better issue legal notice against him by giving legal warning whatever consequences to be faced in future, i.e, criminal case prosecution. And give paper publication regarding your property by divulging all material facts to public awareness.