First you need to check property documents. The land which your friend's and family has whether it's self earned or inherited property from ancestral and ancestral law. 

The friend belongs to which religion according to that law will be applicable.

Plus any government acquisition, reservation, gifted etc on that particular land. 

He couldn't sell the land means whether it's forest land or gifted by government to adivasi rules etc.

First check above details with him then we will be able answer your question whether to purchase the land or not or to become trust and apply trust law.

1) He can execute gift deed in your favour for 50 per cent share in land 

2) in alternative he can execute sale deed for 50 per cent share in land 

3) if it is large piece of land it can be sub divided . 

Execution of instrument of trust by your friend will only confer on you right to derive benefit out of half of property You will not be owner. It is not transfer of property to you. Ownership remains with your friend. If that is ok, your friend can execute an instrument of trust in your favor. This is how “trust” is explained under Section 3 of Indian Trust Act… A “trust” is an obligation annexed to the ownership of property, and arising out of a confidence reposed in and accepted by the owner, or declared and accepted by him, for the benefit of another, or of another and the owner.

Your friend's contention may not be correct legally. When he can sell you half his property, it has to be subdivided. There is no other way. Please engage a lawyer to scrutinize the documents in possession of your friend to check his title and ownership over the property and also to verify if there is any documentary limitation supporting his contention.

He can sell you the part of his property

If you want to have clear and marketable title to the property you desire to acquire from your friend, then he should execute a registered sale deed in respect of the portion of property he decides to part with in your favor.

He has to demarcate the boundaries of the portion of property he now sells to you properly with the help of a surveyor and should hand over possession of the piece of land sold to you.

There is no question of making you a trustee to the trust he may form becasue this will not give you rights over the property in which you intend to invest now.