If possible, I'd just avoid driving with her. The window opening thing was a good idea.
My thought is, what's going to happen after you tell her you're sensitive to strong smells and can get carsick? Are you going to ask her to take the air freshener down (picturing it hanging from the rearview mirror) and put it... where? Unless she puts it in the trunk you'll still probably be able to smell it.
And, her removing it at the start of one ride is not really going to significantly reduce the smell, I wouldn't think. So in order for you to ride in her car at all comfortably, she would have to throw away the air freshener several days in advance and possibly let the car air out extensively, and then never get another air freshener. And that seems to me too much to ask, unless maybe several of you are going on a long "office road trip" to a conference, or she is going to be your long-term carpool partner, or something like that.
Sorry, not trying to criticize, I hate those strong, artificial scents too and would probably be miserable also. I'm just not sure what outcome you're looking for by telling her.