We had a similar situation when we picked up our son in South Africa a few years ago. Can you get a better fare if you buy round-trip for your missionary starting in the Netherlands and just don't use the back end?

The biggest problem we faced was making sure that our return flights matched a flight we could get our son on. I travel a lot (closing in on my second million miles). I have booked my own travel for the last 25+ years. But this was a case where I gave up and used a travel agent to make sure we all got home together. You might want to consider calling someone like Morris Murdock Travel. (By the way, I am not a travel agent and do not work for Morris.)

Good luck to you!