WARNING: NSFW. I am fairly new to Reddit, so apologies for any mistakes. I would also like to preface this by begging you to not make any lewd comments about my daughter. They would be wholly unhelpful and very upsetting to me.

Background: Husband and I happily married for 21 odd years, he was raised Christian, I was not. We have three wonderful children - R(20F), M(15F) and L(7M). I would say we haven't had too many issues, apart from R being diagnosed with minor depressive disorder and L being diagnosed with stage three (severe) autism. This had an impact on our family dynamic and marriage obviously, but we've worked through it over the years. M also has an odd condition where she falls asleep incredibly easily. This has led to fairly minor issues such as falling asleep in class, on public transport and other such places. We have visited a few specialists about this, but it really doesn't seem to be anything of concern. We, as a family, visited therapy for a few months as a trial, but it really just didn't work at all. R was very opposed and stayed defensive for most of it, and my husband and I already talk outside of therapy, so there wasn't much hidden or to talk about.

So, two days ago, my husband, L and I went shopping for a few hours. R was studying at a local library, so M was home alone. (This is a regular occurrence and I must stress that she has absolutely never experienced issues of safety, responsibility or otherwise. This system works for us.) When we came back, M wasn't in her room, and the bathroom door was closed. After a few minutes of knocking on the door and calling out, I got a bit anxious and called out "We're coming in, M!" (In my mind, she could've hit her head and fallen unconscious, or something worse.) My husband then opened the door to see... M, lying in the bathtub, fast asleep, phone in her hand, no pants on. Her... Place was angled close to the faucet and was wet, so it was fairly clear that... something had happened. I walked in to check she was okay, breathing etc. After all, maybe she was just practising good vaginal hygiene? I decided to leave her there, and I grabbed her phone to go plug it in, but to my surprise, it was on. It had... a pornographic video paused on the screen. I was fully blindsided by this, as was my husband, who had seen all of it. He got this odd expression on her face, marched past me and slapped her across the face. I immediately cried out and pushed him away from her, but M woke up and started crying. I felt awful so I pulled him out of the room and left her there to collect herself, but I had a feeling it was too late.

Now, my husband is absolutely fuming at M. I have NEVER seen him like this before. He says he's angry about a few things, which I don't quite understand:

Masturbation. He believes its a sin and is absolutely disgusted that she did it in the first place.
Where she learnt it from, apparently. He is now 100% convinced she has or has had a boyfriend (or multiple?) that have taught her to do this. He even implied that perhaps she was in contact online with older men who had groomed her into it.
The bathtub. He is so incensed that she was in the bathtub, a place we all use. I agree a little on this, but just that it's quite frankly a bit icky of her to do it in our family bathtub when we're out. I don't think it's an egregious sin of her to do it there, it's just a bit bleh.
The water bill. He firmly believes that M has done this before, and is now losing his mind over the fact that she must have spent a lot of water on this. I understand this, but I think it's sort of the least important issue at hand.
I fully sat him down and talked it through with him to explain that masturbation is a natural thing and that she has a right to her body, but he's still incredibly upset about point 2. I think I've talked him down a bit, but he is still absolutely enraged at her actions, and I don't know what to do or say..

Meanwhile, M has been absolutely distraught. I've tried talking to her on multiple occasions, but she hasn't spoken to anyone in the family since the incident. The most recent time was a few hours ago, I sat her down and essentially said, I love her 100%, and I don't really mind what she does with her body as long as she is very careful, and I requested that she stay safe, especially when talking to others. She didn't say a word, just teared up and locked herself in her room again. My husband is taking this as a surefire sign of guilt and has taken her devices (laptop, phone) multiple times to see if she has any connections (to boys or men, I think) that he could find. All she really has is Discord and Instagram (that we know of). Her discord is used very sparsely to communicate about a Minecraft server she's on, and as far as we can tell, her Instagram is only used for liking pictures (generally only pictures of food, DIY videos and her friends' pictures). This has further convinced my husband that she has a second account somewhere. I don't know what to say to that, because whilst that is a large possibility, there isn't much to prove there.

R doesn't know the details of what's going on, but I can assume that she's guessed by now, and L has absolutely no concept of what's going on. This is so incredibly out of character from the both of them that I just truly don't know what to do. I feel like a failure of a mother. Therapy didn't work before, so I have no reason to believe that it'll work now, especially with M being so unreceptive. Understand, this is the first incident my husband has EVER acted like this throughout our entire marriage. What do I do? What do I say, to the both of them, to keep my family together?