I would think yes as size isn't a protected characteristic.
However, anyone could wear their clothes, they just might need them altered. Though in the real world of course even if your dd got to interview stage, it would probably be obvious if she's smaller, especially if much smaller, than a size 16 so chances are she wouldn't get the job anyway.
As a plus size with a very slim dd myself I'm torn - genuinely!
On the one hand as a customer going into a plus size store I would feel more comfortable being served by other plus size people.
On the other, even though dd is very slim herself, she is very polite and through having a plus size mum and a VERY plus size best friend (and this is partly due to health issues/medication), she empathises and understands (as far as she can having never been overweight herself) with those who are and would treat customers very well. Indeed she's suggested styles to me that I thought would look awful that actually were very flattering. She has a good eye for that kind of thing.
Also, some would like to see it become a protected characteristic to attempt to end the prejudice coming from some people towards people who are overweight (and underweight actually), so perhaps employers especially ones where their employee policy is so public, need to be aware of that. But then that might kinda be where they're coming from - trying to make customers feel comfy by not being served by slim sales staff who might not be kind.
Dd is very slim, always has been, but is not underweight, the only times she's been underweight is when she's been very poorly. But unfortunately some people see her as fair game for some quite nasty comments.
I have a friend who is underweight through no fault of her own, struggles to maintain the little weight she has (crohns disease) and she also had had horrific comments from complete strangers sometimes.
Very tricky for the employers.