Should I tell her dad Update

I've veen put in a very compromising situation and I need help. I make a lot of posts kinda hust little stories about my boyfriend and his 4kids. They're all by different women by all in his custody. Well pretty much I was helping Maddie his 14 year old with light make up. I ordered her a soap maker because she wanted to make her own soap. I went outside for like 20 minutes and Amazon delivered it. I went to tell her. Her door was cracked and I still should have knocked and she was taking a topless and braless photo... I walked out. After dinner she came up to me to apologized and begged me not to tell her dad. I told her I think this is something I have to tell him. She said her dad will get mad and ask questions anout who she sent it to and I told her she shouldnt be sending nudes to boys. She said it's not a boy... And she's bisexual and doesn't want her dad to know. Her reasoning behind it is confusing to me so maybe you guys can explain. I told her that her dad will obviously accept her and she said he will make it a big deal. I brought up her older brother Max being gay and their dad accepts him. She said it's different. That Max never had to actually come out from because everyone already knew. Max talked about liking boys as a little kid so there was no need to come. It was already known by everyone. In her case she said she has to come out and it will be a big deal and she doesnt want that. Like I said im confused still on the reasoning but i validated her. This is a tough situation. I could use some guidance

Her mom isn't in the picture. She blames her own daughter for her boyfriend sexually abusing Maddie. I think that might be why Maddie became so close to me.... I've never a women talk with someone before. What all should I say?

Update: So I told Maddie I've been trying to convince her dad to treat her more like an adult and the adult thing would be for her to tell her dad. I told her if she didn't I would have to. After a while we went into a bedroom and closed the door and talked to my boyfriend. Now when she said what she did, he did yell and sge started crying so he calmed down and they had a talk. It ended well with a hug and kiss but he did take her phone away until she proves she's responsible enough to have one. But all in all it went well.