Gotta get to bed, but want to just throw out my thoughts. Unfortunately, single mom's who take men into their lives and homes risk harm coming to their children, no matter how sweet or charming the man is. Many cases here have been about step-dad/mom, "live in" (as NG always says) or whatever being allowed to get close with disastrous results.
IMO this boyfriend was twisted and waiting for an opportunity to sexually abuse or act out his fantasy with Hailey. He dated Mom and eventually got live in privileges to pursue his demons. I think Hailey was being abused in the home or he was attempting to. She may have been too embarrassed or felt Mom would not believe her. The drugs he used perpetuated his sick thoughts and gave him the impetus to act on them.
He may have done something harsh physically and sexually - accidentally killed her, or even killed on purpose for his "game." Billie seems sweet and innocent, and wanting to believe her boyfriend would never do anything - how she could really believe it I don't know since he had threatened them in the past.
He probably got home after his act upon Hailey, took more drugs and was wildly hyper as in "deer in the headlight" look.
I do not think Billie took part in any harmful act on her daughter. But she know something, the things she says are strange and indirect in her responses. She may have even been high that night (Sunday/Monday) and has poor recollection of events.
God bless little Hailey.