morally confused UPDATE

I'm posting anon because I don't want to get any negative back lash on my future posts. Today while cleaning the bathroom after stocking toliet paper tampons, etc. I opened my boyfriends 19 yr old daughter travel tampon case to put some tampons in it. (I was not doing it to be snoopy I was trying to be helpful, I know how it can be running out) anyway low and behold there is a pregnancy test. Unwrapped. I know their usually sold in two packs so I'm thinking one was taken. I know we get our periods at the same time and she had hers last month around the 20th as did I. I'm not sure if I confront her about this and let her know about low income costs where her mother won't have to know. Also tell her how her dr wouldn't tell her mother (she's on her mothers insurance) or should I tell my boyfriend. Or should I keep my mouth shut and wait till later this month. We share a bathroom so I'll know if she gets her period or not. (I do know she could just have the test but it's unlikely) 
UPDATE: maybe I should of meantioned that her father and I have been together for 8 years. Her and I have a fantastic relationship. However her and her father live in my house that I pay the bills to. If you want to state that she's grown maybe you should understand that she pays no bills, her cell phone is in my name and I pay it. She has a job for three hours a day weekdays and school year only. She "forgot" to register for classes this year. I give her money here and there for things. I pay for all the food she eats all the food her boyfriend eats. So no she is not grown. I will not tolerate this child trying to get pregnant on my house. Absolutely not, she has a bright future if she bucks up. We actually had a great conversation, in which I told her I will not tel her father, I understand scares happens but she needs to be safe. I told her if she ever needs anything I will not judge and I will help her to the best of my abilities. (I am a nurse) I also explained with her history of eating disorders and depression she needs to be extra careful because an unplanned pregnancy can exacerbate those issues that she's worked very hard to get under control.