I've booked separate inbound-outbound tickets before when I do open-jaw and fill in a leg on a LCC. I've been asked questions and required to show onward visa clearance.
I've also checked in using automated kiosk/phone for a domestic-international itinierary, and then called up to the counter at the gate because the airline needed to check my passport. It's not a commentary on my nation of origin.
My personal opinion, based on what the OP provided, is that the OP's acquaintance was not aware of the necessity for follow-up questions given his travel arrangements and felt hostile toward airline personnel, and then the situation escalated.