How should a young man act who discovers that a quarter of the money for his tourist trip comes from a bank loan, taking into account that his food, accommodation, and clothing during the week-long trip will be from this mixed money, and that he has no other source of income?

Borrowing with usury is a major sin that incurs curses and obliterates blessings, and your father must hasten to repent to Allah. As for your benefiting from your father's money borrowed with usury, there is no harm in it, because the prohibition is related to the borrower's responsibility, not to the loan itself.