What does Srila Prabhupada say are a husband's duties to his wife?
Hare Krishna! Thank you for your question. To find out everything Srila Prabhupada has spoken on this matter you'd have to read all twelve Cantos of Srimad-Bhagavatam! However, I can give you some general instructions in regard to a husband's duty to his wife. Below are some quotes from Srila Prabhupada's Bhagavatam:
From the story of Devahuti and Kardama and their son Kapiladeva, from the 3rd Canto of the Srimad-Bhagavatam:
"It is the duty of the husband to liquidate his debt to his wife. The wife gives her sincere service to the husband, and he becomes indebted to her because one cannot accept service from his subordinate without giving him something in exchange. The spiritual master cannot accept service from a disciple without awarding him spiritual instruction. That is the reciprocation of love and duty."
"It is the duty of the son to take care of her to the best of his ability so that she will not feel separation from her husband, and it is the duty of the husband to leave home as soon as there is a grown son to take charge of his wife and family affairs. That is the Vedic system of household life. One should not remain continually implicated in household affairs up to the time of death. He must leave. Family affairs and the wife may be taken charge of by a grown son."
And, from the Bhagavatam's section on Lord Ramachandra:
"One who strictly follows religious principles must not neglect to provide all facilities for the complete protection of his wife. There may be some suffering because of this, but one must nevertheless endure it. That is the duty of a faithful husband. By His personal example, Lord Ramacandra demonstrated this duty. Lord Ramacandra could have produced hundreds and thousands of Sitas from His pleasure energy, but just to show the duty of a faithful husband, He not only rescued Sita from the hands of Ravana but also killed Ravana and all the members of his family."
I hope this is helpful. Mostly, the husband is responsible to give his wife spiritual guidance, and protection - in the material sense - by providing for the needs of the family and the wife herself.