I would say just ask her nicely but simply.
Literally just say, "I am sensitive to the smoke. Do you mind smoking outside when I am around?"
The less dramatic you make it, and the less you lean into things like saying how bad it makes you feel, how bad it is for you, etc (all of which will just make the other person feel defensive instead), the better it will go. Phrase it as a simple request, like if you were asking them to get you a glass of water (or something else simple like that).
Now, if you ask her nicely, and she says no or makes a big stink about it, then she is not respecting your requests and you would be in the right to avoid her.
It isn't fair to just stop visiting her without asking nicely first. You have to give people the opportunity to do the right thing.