I really hope you’re joking. We are not educated in all the ways people can be allergic to things and you are truly pushing a barista to experiment with a customers allergy.
$1. Do your research before you make assumptions and play with the well being of other people due to your own bias against them because it doesn’t make sense to you.
Even with the possibility she is making it up, doesn’t matter. She wants non-fat, she can tell the difference, and enough baristas have messed it up when she asked for nonfat she had to resort to claiming it was an allergy to be taken seriously she would like what she ordered. Just as most people who order no foam lattes don’t care if there’s a little bit of foam, they got a cappuccino instead of a latte enough they decided best way was order no foam. Go ahead and ask your next no foam customer if they’re okay with a little foam but they had experience of too much foam before. 9/10 will say yes, I know because I ask them all.
OP, id encourage you to frame this question to her in a polite answer I bet you’ll get a reasonable justification for her order. Something along the lines of “hi! I hope you don’t mind me asking but I’m curious, I have many customers who are allergic to dairy in general but you’re the first I’ve encountered who is okay with nonfat but not other types. You don’t have to share or anything but it would definitely help me to understand in the future!” I bet you’ll get a response, even one that admits the second scenario I presented.