It doesnt necessarily have to be native. It's called dress-down day meaning, to be on any casual outfit which could be in the form of native or other causual outfits like jeans and t-shirts or sleek dresses.

Now, it's not all companies that dress-down on Fridays and for those who do, it's not mandatory. you can choose not to and no sanctions on that.

Fridays seems like a less busy and short day of the week ( for some, not all companies) ; also the beginning of the weekend. So, I guess dressing down is just a way of easing the previous days stress and saying TGIF to Kickstart the weekend. My thoughts though.

For me, I do jeans on tops or native depending on my mood. Other times I do corporate. So, no biggies.