It's hard to watch someone you care about make poor choices. It may be even harder to get through to her, especially if she feels strongly enough about this guy that she's willing to be dishonest about their relationship.
Since your friend is trying to hide her dating, she probably already knows that it isn't right. Let your friend know that you love her and care for her, even if you don't agree with her choices. Don't do anything to encourage her behavior, like lying for her.
In For the Strength of Youth it says that "a true friend will encourage you to be your best self" ("Friends," 12). Perhaps the best way to do this for your friend is by example—by being your best self. Stay true to your standards. Invite her to activities that show her that she can make friends and have fun without dating. Your friend will see the benefits of waiting to date.
Pray for her and ask Heavenly Father how you can best help her. If you feel that she would be receptive to it, try discussing her choice to date before turning 16. Tell her why you think youth are advised to date only after 16. Encourage her to pray about her current choice and where it may lead her. Bear your testimony that waiting to date will keep her safe and clean.