I would not want a bare bummed (partiually, or otherwise) child sitting on my carpet or couch, so treat is almost as a sanitary issue. Kids that age are not concerned about hygene (they may not wipe very well, if at all) and if part of her bum is hanging out, that would gross me out, apart from any 'mature' gestures she may display.
I agree with a new dress policy about shorts or tights being worn under skirts. maybe casually mention to your friend that you are putting this into effect to mimic the school policies. I wonder if she would take that hint and start appropriately dressing her daughter, even though she is technically not in your care as a charge.