My friends is obsessed with get boyfriend?

My best friend is going out with a guy and he is a really nice guy but my friend is obsessed. Everything is about him and I know that they are sexually active! After they had sex she is always with him! I tried to talk but she denied it and got mad. Her grades plummeted and her relationship with her family and friends have disappeared! All her "friends" talk bad about how she is needy.She talked on the phone won him for the whole night of our sleepover and forgot i was there. I don't want her to be mad and I don't want to lose a friend. Please help me figure out how to get my best friend back because I am her only real friend now and only connnection to getting her grades back up.