Is a husband who works in domestic tourism, booking rooms in hotels that may contain alcohol or feature mixing and nudity, considered sinful, knowing that he only books for people who have a marriage certificate?

Working in tourism organization and hotel booking is permissible in principle. It becomes unlawful if it involves assisting in forbidden acts, such as mediating the rental of places for drinking alcohol, fornication, or gambling. However, mediating the rental of hotels for those who have a marriage certificate to stay there is permissible. If the tenants engage in something forbidden, there is no sin upon the landlord or the mediator, because the rental was not for an unlawful purpose but for a permissible one. The company owner should strive to adhere to what is lawful and avoid what is forbidden and doubtful.