Maybe the position that she's in is compressing the nerve that runs down her leg. You could try something different.
However if she's happy then there may not be a need to change anything.
I wouldn't worry about it. If she likes it and she is in no discomfort, let her feel good.
The numb legs can be due to a pinched nerve. I notice that sometimes my left upper thigh starts pinching then numb while having sex and I figured it was that i needed to streach more. Hope this helped