I sent nudes to my best friends boyfriend

I'm not going to explain how it all happened but in the short version I sent her boyfriend nudes. My best best friend. They are official but you would say they were dating. He asked and asked and I fell into it at 2 a.m. Should I tell her? Or just block him and move on. She's like my sister I can't lose her. I'm a horrible person. I know. What do I do?