My Friend Is Trying To Steal My Boyfriend...

Ok, so I was friends with this girl. Lets call her L. Me and L have been friends since 2 months before we started Pre-K. She had mild family issues and leaned on me and my family for support which we accepted. She basically lived with us from Friday to Sunday and we loved each other like sisters. She loved my parents and we basically shared them. She even got along with my brothers. She was the sister I never had.

We went on as best friends up until a few months ago. L started hanging out with a group of people that I dont associate myself with. This group is into drugs and theyve gotten in trouble with the police multiple times.

First L would kind of go hang out with them during breaks or eat lunch with them instead of me. But then it started to be all the time. After a while L started ignoring me completely. We stopped talking, having sleepovers, and basically acknowledging one another.

Fast forward 7 months. I have a boyfriend now. She starts talking to me again. And heavily flirting with my boyfriend. DESPITE: her knowledge of us dating.

He was completely against the flirting. He even got mad and jerked his hand away when L reached for it.

I know he wont cheat on me but L is betraying me. She knows that me and him have gotten pretty serious. And everyone knows that he wants to marry me because hes said it(were 18 so future is everything).

Im just really mad that she thinks she can just walk back into my life to try and take my boyfriend that Ive been dating since before she started straying away from me as a friend.

Advice anyone?

What should I say? What should I do? What are my options here?