I wouldn't be happy about that at all. I guess I would be working hard on my mother/daughter relationship and making 1 to 1 time where possible, even if its just a hot chocolate and a natter and the focus would be for her to trust me as a mum. Building and holding her trust is really important. I would also be looking at all the older kids responsibilities around the house and trying to pull them all in as a tight knit group. Have some agreed 'contracts' (rules) that they can all help decide on and set.
I would be having a word to his parents (assuming he is the same age) and let them know that she was invited to his bed at 11pm. Let them know some of your concerns and let them know you feel your daughter is 50% responsible for her part.
I don't envy you. Its a difficult time for you!