The wedding guest should consider herself much-loved and fortunate to have gotten the invitation to stay at the wedding venue, and to accept it and enjoy it. While she certainly doesn't want to take advantage of a good friend, it would be very rude to insist on paying for her room at their daughter's wedding at this point, especially since she's already offered, and they've already declined.
It's the bride's privilege to ask her wedding party to dress a certain way for her wedding. It's tradition. She can tell them what jewelry and shoes to wear, too. She can even ask everybody to wear their hair up, or down, if she's super picky. But she absolutely, positively CANNOT ask anybody in her wedding party to alter their physical characteristics in any way.