The story about this being an intercept on the airport wifi is bogus. All communications between a device and snapchat is encrypted (SSL). There is no way the airport can intercept this without having the Snapchat certificate and key - which of course they don't.
There are only two options here: either (1) one of those "friends" played an active role here, or (2) Snapchat itsself actively monitors all communications on their platform and flagged this and forwarded it to authorities. Which, if true, is pretty scary - a private companiy which is monitoring private communications between regular individuals for "keywords", and reporting those individuals just based on what they tell each other, context not taken into account. Good reason never to use Snapchat if that is true.