Can a wife be a mahram to her husband, such that it is not permissible for her husband to enter the house of her mother's friend when her adult son is present and her husband is absent?

The presence of an adult son prevents unlawful seclusion (khalwa). As for the presence of another woman, there is a difference of opinion, but the preponderant view is that it prevents unlawful seclusion if there is no suspicion. There is no blame on your husband for refusing to enter your mother's friend's house, even if seclusion is eliminated, because accepting an invitation is not obligatory.