@Kat11165 One air freshener is one too many. As a host, you have to be aware that not everybody likes scents, and a scent that appeals to you may not appeal to others.
Had you asked me as a guest if I had "allergies" without being specific, I would have said no. (I do have seasonal allergies, but there's nothing you can do about that.)
HOWEVER.....I can get really sick from air fresheners. Depending on how strong they are, and what scent, I can get a really bad headache and feel generally unwell.
So A: don't use air fresheners in your guest space, and B: if you're going to use them, check after your cleaning crew to make sure there aren't "too many."
She sounds somewhat difficult, but I'm sympathetic to her situation. She came to your space, and it made her sick. I kind of understand her not wanting to stay. It does take several hours for a strong smell to clear an area, and in your space it has likely permeated the bedding, furniture, etc. You might have wanted to refund her something for the trouble, and perhaps warded off the bad review/rating. Lesson learned, I guess.
I actually think her review was very fair, since she compliments the space and then says specifically that the air fresheners were the problem. I don't see it as retaliatory, as her experience wasn't great. I think when you ask about "allergies" you need to ask specifically about air fresheners and scents. Guests might think you are asking about cats, dogs, or peanuts.