Does Pvt Ltd Company is a Mandatory requirement to do Medical Software Business?

Dear Members,
I have a query
One of our clients is in the business of providing Software services to Various Hospitals In India. 
His business is to purchase software from a foreign Company and to sell it in Various hospitals in India and to provide annual maintenance service for this software. 
Currently, he is doing this business as a Pvt ltd company. But now he wants to close the company and to open a proprietorship for the same business.
My Query is can he do the business of medical software as a proprietorship, or is it necessary to it do as a Pvt Ltd Company?