Does the airline or airport offer a (probably reserved and paid in advance) escort service that has the capacity to help them in their own language? Can you arrange for a bilingual FA to stay with them through immigration and customs (probably not)? If they speak a common language (or common language on the flight), another passenger on their flight might be willing to help.

The mention of ESTA suggests that they're coming from a developed country, so an extremely exotic language is unlikely, but it would help to know the airline, route, and language.