Can anyone help?

So, last night I had my friend over to help me with a project for a class that we share, and she told me that her abusive brother was staying at her house tonight, and didn't wanna go back, so I offered to have her stay over so that she wouldn't have to go back, completely forgetting to even ask my mom, and mind you, I was alone in the house that night. My mom came over in the morning, and it completely reminded me that I totally forgot to ask, and neither of us said anything because my friend was there, but now my mom is saying that I'm lying, and I only feel guilty about it because I got caught, which is not true, I really did mean to tell her, and I genuinly do feel bad that I forgot to ask. She is also saying that it was a total breach of her trust, which I totally understand, but she won't listen to me when I tell her the truth about it. Does anyone have any advice?