Do your friends have a yellow fever certificate? Here's what my consulate's website says about this:

Yellow fever vaccination certificate, as applicable, i.e., if applicant traveled within the last 90 days to any of the following countries: Angola, Benin, Bolivia, Burkina Faso, Burundi, Cameroon, Central African Republic, Colombia, Congo, Côte d’Ivoire, Ecuador, Equatorial Guinea, Ethiopia, French Guiana, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Guyana, Kenya, Liberia, Mali, Mauritania, Niger, Nigeria, Panama, Peru, Rwanda, Sao Tome and Principe, Senegal, Sierra Leone, Somalia, Sudan, Suriname, Tanzania, Togo, Trinidad and Tobago, Uganda or Venezuela.