• Ensuring their passport is valid. According to gov.uk¹, some airlines or cruise operators require at least 6 months validity in a passport at the return date.
• Advise to take a photocopy of their passport and certificates and also store it digitally. Leave a copy with friends and family in case of emergency.
• Take enough money to cover for any emergencies, try and keep this money in different places.
• Take a money belt and avoid displaying cash in open public. The same goes for any expensive digital equipment such as cameras. 
• Never leave personal valuables in a hotel room, keep them locked in a safe (if available)

^{[1]assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/752931/Checklist_for_Travellers_301018.pdf}