Clearly she made it public to the people she works with. She wasn’t surprised that her boss knew - because she has obviously discussed it with her boss. If it were private she would not have discussed it with her boss. So it is public, yes?
Well, yes, it is just an opinion off the internet. But then every person who posts an opinion here, including you and me, are just opinions off the internet too. The critical question you haven’t answered, is WHAT LAW was broken?
I don’t think he had any right to do this and stepped over the line. However if you still got the new job I would let it go with the email that you sent for the explanation. I personally like people around me knowing I am diabetic b/c of potential help needed. I never disclose it at an interview but after the probation time I typically try to let them see my pump and act like nothing has ever happened. When they ask I just tell them like it’s nothing and never act like I tried to hide it. 
Hope you like your new job.
I would be Peed off about this,I told my store manger and supervisor about my diabetes,My supervisor has it to and there is a few of us in the store,it is a evasion of privacy,go to the department of labor,or human rights tribunal,
Not being funny here but, I like people to know that I am a T1 as it could save my life. So if in-fact you go low at work, they won’t think that you are drunk or crazy but rather have glucose tabs on hand. It could also be looked upon as a safety issue and the isn’t private or protected. Nothing to be ashamed of or worried about. If they had not hired you, you could have went in there with 40 cans of spray paint and decorated the place and then blamed on a low sugar state and the doctors would back you up. Don’t get any ideas now 