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
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.