Is a security guard sinful if he allows a young man to enter with a girl, whom he claims is his wife, despite the existence of internal regulations prohibiting mixing, and his knowledge of the falsity of his claim, for fear of problems?

The company's regulations do not permit mixing (between unrelated men and women), which obliges the security guard to prevent it. One must adhere to the limits of Islamic law. If the employee knows that the woman is a stranger to the man, he is not permitted to enable them to engage in unlawful seclusion (khalwa), even if a false oath is taken. Fear of work-related problems is a relative matter, and its ruling varies, but the fundamental principle is the impermissibility of allowing such individuals to enter, especially given the company's regulations that prohibit it. They should always be prevented as long as it is possible to do so.