That is not a risk for herpes unless she specifically had a herpes lesion on her
buttocks. People do not shed virus without symptoms from their
buttocks - the
skin is too thick there for shedding. So if your only contact was penis shaft to buttocks without a lesion, there is no risk of herpes here.
If you were to have a lesion, yes, it would be on the penis, not the thigh at this point.