Hi Thomas,
To get to Bocas by land is very easy. Your friend will pass through the Sixaola border crossing into Panama. Probably the easiest way to do this is to sign up for a shuttle or tour in a place like Puerto Viejo de Salamanca on Costa Rica’s Caribbean coast. They’ll get him onto Bocas del Toro quickly and easily, shepherding him through the border crossing and all the paperwork. He could also take the bus or taxi to the border and manage the crossing himself, then catch another bus/taxi and then water taxi to get to the Bocas Town.
As far as documents go, it’s important he have proof of onward travel out of Costa Rica within 90 days of entry. So a print out of his plane ticket out of Costa Rica. Costa Rican immigration will definitely want to see this when he re-enters Costa Rica. Panama immigration will probably want to see it when he enters as well. Other than that, the passport, of course.
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