Answer by taking a quote from the following article:

Devika Rani Chaudhuri, usually known as Devika Rani (30 March 1908 - 9 March 1994), was an actress in Indian films who was active during the 1930s and 1940s. Widely acknowledged as the first lady of Indian cinema, Devika Rani had a successful film career that spanned 10 years. Born into a wealthy, anglicized Indian family, Devika Rani was sent to boarding school in England at age nine and grew up in that country. In 1928, she met Himanshu Rai, an Indian film-producer, and married him the following year.

Najm-ul-Hassan was also Devika's co-star in the studio's next venture, Jeevan Naiya. The two co-stars developed a romantic relationship, and during the shooting schedule of Jeevan Naiya, Devika eloped with Hassan. Himanshu was both enraged and distraught. Since the leading pair were absent, production was stalled. A significant portion of the movie had been shot and a large sum of money, which had been taken as credit from financers, had been spent. The studio therefore suffered severe financial losses and a loss of credit among bankers in the city while the runaway couple made merry.  Sashadhar Mukherjee, an assistant sound-engineer at the studio, had a brotherly bond with Devika Rani because both of them were Bengalis and spoke that language with each other. He established contact with the runaway couple and managed to convince Devika Rani to return to her husband. In the India of that era, divorce was legally almost impossible and women who eloped were regarded as no better than prostitutes and were shunned by their own families. In her heart of hearts, Devika Rani knew that she could not secure a divorce or marry Hassan under any circumstances. She negotiated with her husband through the auspices of Sashadhar Mukherjee, seeking the separation of her finances from those of her husband as a condition for her return. Henceforth, she would be paid separately for working in his films, but he would be required to single-handedly pay the household expenses for the home in which both of them would live. Himanshu agreed to this, in order to save face in society and to prevent his studio from going bankrupt. Devika Rani returned to her marital home. However, things would never be the same between husband and wife again, and it is said that thenceforth, their relationship was largely confined to work and little or no intimacy transpired between them after this episode.  Despite the additional expense involved in re-shooting many portions of the film, Himanshu Rai replaced Najm-ul-Hassan with Ashok Kumar, who was the brother of Sashadhar Mukherjee's wife, as the hero of Jeevan Naiya. This marked the debut, improbable as it may seem, of Ashok Kumar's five-decade-long career in Hindi films. Najm-ul-Hassan was dismissed from his job at Bombay Talkies (this was the period in which actors and actresses were paid regular monthly salaries by one specific film studio and could not work in any other studio). His reputation as a dangerous cad established, he could not find work in any other studio. His career was ruined and he sank into obscurity.

Who is he?
Najm-ul-Hassan was also Devika's co-star in the studio's next venture, Jeevan Naiya.