tell her she's 16 and he's 18 and it's illegal, tell her NO.
She's expecting you to tell her something, she doesn't need a best friend, she needs a parent, you're allowed to tell her what she must do, she's a minor, she isn't an adult yet, speak to her.
There is no excuse for teenage pregnancy, it's not just the kid's fault, it's the parents fault. Introduce some structure in her life and set some boundaries, seriously what else are you going to give in on? It's ok to drink & do drugs but you must do them in my home, I won't allow you to do at someone else's home or out on the street. WTF?!