I know nothing should surprise me, but I was in shock when I read your post. I thought of my parents who are in their late 70's and have heart conditions. My father has stopped flying, but my mother flies a little and almost always in purchased first or business. She's nervous enough about flying, and she would have been scared if someone had grabbed her boarding pass. She is not able to walk very fast so we usually request a wheelchair for her. My parents have never flown enough to have elite status and wouldn't have known what 1K meant.
UA handled your situation well, and I hope security was called. I'm glad that you had an enjoyable flight.