Well he is not one. He cannot read minds. He doesn't know that this girl before him is innocent. He doesn't know that Gauri was sold to Takur. He doesn't know that Gauri never wanted to get married to Takur. He doesn't know that this girl was chained. He doesn't know that she was going to be married to three guys. He doesn't know that she ran from her wedding to save herself. He doesn't know that Thakurayin committed suicide not because of Gauri but because of her husband.
But he saw with his own eyes the Thakurayin crying about her Sautan and trying to immolate. He saw the Takur saying the girl gave consent for money. He saw Gauri hugging the Takur. He saw her smiling at the Takur. He saw her dancing at the wedding. He saw her trying to touch him in front of other men. He saw the Takur giving her gold. He saw her running from the wedding. He saw the villagers accusing of being the murderer of Thakurayin, the same thakurayin who said she is going to immolate because of her sautan to him.
To add to all this the woman's name is Janvi, like his mother who almost had the same fate because of the third woman Swetlana. And to him Gauri is Swetlana. I believe it was great of him to even save Gauri when he hates her like hell.
Peace.