crush advice

I am 13 I have liked this boy for 6 months and when I started to like this boy I told my friend about him. she goes to another school and lives 45 minutes away and I told her who he was thats what girls usually do, and I just recently had a Halloween party/ sleepover the other night and all a sudden she was staying in a room and messaging him and flirting with him she also was sending photos of her and her clevage. and honestly I was hurt by that I feel betrayed and heart broken I never would of thought my closest friend would do that to me I'm shocked she also wants to meet him at my park down the road with out any one else, I don't know what do do I havent told her how I feel because I don't want to lose a friend over a boy you know. And today in sport, my crushes best friend came up to me saying about this girl she was flirting with him my crush does she like him he asked,  and I said I don't know but she does I'm positive she was looking at love quotes and crying from love songs she also found a quote saying, I've never met him in person or don't really know him but I've fallen in love with his messages. I mean it's obvious she likes him, I feel really hurt and she also said that she wants to have a sleepover this weekend to met him so she's basically using me because he lives around my neighbourhood so she can met him and get with him I don't know what do do. Help me please