There is a law that says you can't go over someone else's house?
That's a very stupid law.
Even if she did spend nights at Bilawal house, what does it have anything to do with the law? There is no law in Islam or anywhere for that that matter that says you can't go over to someone else's house.
And you're the one here assuming that she slept with him at his house. Exactly back to my point, you guys just can't seem to mind your own business. There are mullahs running masjids who sexually abuse children. I've seen it happening with my own eyes. Go lecture them about Islamic Laws and Humanity