Honestly, in that situation, I would leave her behind and find someone/something that could help me, just as you did. But what do you say going forward?
I love the idea of a note: it's informative and non-confrontational. It's effectiveness, sad to say, is all predicated on her willingness to learn.
When I first became a T1, lo those 50 years ago, I was told by my school counselor that I shouldn't tell people I was a diabetic because people would consider it a "character flaw." I had a friend that constantly heckled me because I talked about my diabetes freely, just as he would talk about being gay. I had a boyfriend whose partner at work died over the weekend from undiagnosed diabetic hyperglycemia. And just a few weeks ago, I had a severe hypo that left me unconscious - and my friend wanted to "give me an insulin shot" to bring me out of it. (Thank God he called the paramedics instead.)
Some will learn to listen; some will never learn. The most important thing you can do is discern the difference.