I slept with him when I wasn't suppose to

So I'm 18, and my friend/very close partner whose 19, has flown up to see me (he lives about 10 hours away). And last night we slept in the same bed, but i guess a 'rule' that hasn't really been used before is that we were not suppose to sleep together. Long story short, my mum is extremely mad at me, especially because I told her we were only friends. But I told her we were just up all night talking, which was mostly true,  it I don't think she believes me. Please help. How do I make it less awkward for him and make it up to her. I feel like shit now.. and he's staying for another 4 days