Unwanted d*ck pics from my best friend’s boyfriend

This morning I was getting my little sister ready for school and a Snapchat loaded from my best friend of 7 years. I thought maybe she needed someone to talk, seeing as last time we talked she told me her and her bf deleted social media for each other, so I loaded it, and it definitely wasn’t the case. What loaded was a picture of her boyfriend’s dick and my best friend naked in the background, giving him head w the caption of “getting this big dick” 🤦🏼‍♀️🤮 She has been dating him for only 3 months but have been sleeping together for longer, so it’s not that I’m surprised they’re having sex, but it’s not like I wanted to see it. I’m so confused on which of them sent it, why it was sent, if my best friend knows he’s actually sending it, who all it was sent to, just so many questions. I texted my best friend’s number just asking if she approved it before it got sent around, but she hasn’t said anything. idk what to do or how to avoid that from happening ever again