Selling a share from jointly owned property without co owner's consent

I have bought a house jointly owned between my friend. It is tenancy in common . There are no other specific wordings about the property disposal in the purchase document. The property has a pathway dispute with our neighbor and we have filed a case against him. Now my friend wants to sell his 50% share of the house to the neighbor and going to come out of the case. I am worried that if my neighbor buys 50% share from my friend then my share would be in trouble and the case becomes weak. Also I’m not in India now.Is there any way to stop my friend from selling his share to that neighbor?please advise.