Should i IGNORE THIS FRIEND?

I have a friend who is also a cousin of mine. She went behind me on something we planned to do together. Its a boyfriends thing. I know everything, but she does not know that i know everything. so i asked her if it was true. But she denied it. She denies everything. She keeps lying to my face when i knew the truth and asked her. Because she thinks i don't know anything and that i don't have proof. But i do. I found out everything because i knew her facebook pword and i got online from her id and that's how i knew the truth. But she denied everything because she doesn't know i know her pword. I secretly peeked and saw her typing the pword. Thats how i knew. She is the kind of person who turns to friends when she has no one else to hang out with. She is very selfish and thinks only about herself. What should i do? Truth is, her betrayal has really hurt me. And i frankly believe that i deserve better. She has lost all of her friends because she is selfish. What do you think i should do? Should i ignore this cousin of mine? Give me some tips on how to ignore her and make OTHER new friends PLEASE??