That lady said a horrible thing to you which was totally out of line, but the more I think about it, the more I feel sorry for her. She's probably at her wits end.. I know at my old job I worked with alot of skinny girls and they were ALWAYS giving me weight loss advice even when I wasn't asking for it, and wasn't even dieting at the time!!! I never said anything nasty to them, but to be honest I was actually really offended by their efforts to help. I wasn't ready to lose the weight then and I felt like I had to hide what I was eating from them all the time in case they started with the "tut tut, you shouldn't be eating that!" routine.
It's a totally different situation, but in a way it helps me relate to the angry lady. If you could stand it, I'd try again with her. Look out for her next time, and say hello again, and this time mention that you know more about the situation than she might think.
Tell her you won't offer any encouragement from now on, but should she feel like she needs some, she can give you a nod and you'll give her some kind words.
You might get her to see the mistake she has made and see that she's been terrible to you, and who knows, it may help her on her journey!
Of course.. she could just be a cow
All the best..