You might not understand why a trip to Walmart makes your missionary a little sick. You probably won't understand why they are crying so much, especially since they are FINALLY reunited with their family and friends. It might confuse you when they stop mid-sentence and quit trying to explain something.
Hang in there with them. Help them by curiously and gently asking things like:
What's upsetting about that for you?
How did that make you feel?
That seems super important to you. Can you help me understand why?
What was that like?