my knees are right under my bars (i have drop bars, so they arent high up as opposed to risers or something) and sometimes my knees get higher...
it's all about where you feel comfy skidding. watching a lot of my friends there's no 'real' spot for your legs to be while skidding... just balls to the stem and whatever you do with your legs is different person to person...
i find it a lot more comfy and in more control when my leg for my back foot (the one pushing back making the skid) is slightly bent, but have seen many skid with their leg straight...
do you feel more comfortable skidding on your friend's bike?