You mention that she does this after 6. Assuming she has been in the office all day, it's likely she's waiting until she thinks it's ok to smoke. All you need to do is convince her to wait a little longer. Perhaps she waits until the boss is gone and she won't be caught, or until one particular employee who she knows has health issues with it is gone. You need to know which it is, and this is best done by asking her.
The moment you smell that smoke, stop by her office or use a chat app if that's what your office uses, and ask "how come you smoke in here in the evenings?" If you want, you can add "it really affects my breathing" or the like. Depending on the answer, you could then say something like
Well, if you smoke while I am here, I'm going to email the boss and report it, because I was told the boss had arranged for there to be no smoking when I'm working
Well, just because X has gone home doesn't mean nobody is hurt by the smoke. I am, for example
Either way, try to end on a positive note like
I would really appreciate if you could wait until I am also gone. If you like, I can let you know when I'm headed out.
And then smile. You have control here because you know the boss doesn't want this happening. You're offering her a way to get some of what she wants (smoking after you leave.)
Should you mention the heart condition person? I wouldn't at first. It could trigger a sense of being ganged up on. If your approach works, you can just repeat it for the other person.