Dear "Conflicted Friend"
Please please please, for the love of your social life and everything young, fresh and sane, do not take Amy's advice.
I know, I know, she has everyone's best interests at heart, and is trying to protect your "friend".
But apparently Amy has never been a 15 year old, and certainly not a 15 year old snitch.
She is half right, if you follow this advice your friend will be angry. And she will hold it against you: for 4.5 days GIVE OR TAKE A LIFETIME.
In fact, there are high odds, that somewhere between those 4.5 days and the many additional years she will choose to never trust you again she will make note to any one who inquires of your rift that you really aren't to be trusted with any personal information that they wouldn't want their mother to know, lest you become "friends" with their mothers as well.
It's not that teenagers should and must keep secrets from their mothers. It's that they do. They very simply do. It is part of exercising their independence.
Additionally you have no idea WHY your "Laura" has not chosen to share this information with her mom, but as a friend I'd suggest you give her the benefit of the doubt and assume she may have a good reason.
Unless I have been unclear, to take information that has been confided in you in trust and to pass that information on, specifically to a source you KNOW your friend would prefer it kept from, is a BETRAYAL OF TRUST.
And yet. And yet you feel you are betraying the mother's trust and I get that.
So what to do...
Perhaps going to this friend, and describing your predicament is a good place to start. Trusting your friend's judgement is an integral part of the relationship. Letting them know that you feel akward knowing information that you think should be shared with her mother might open up some channels currently closed. I would even encourage her to update her mom on her sexual status.
And if you feel "Laura" is actually engaging in dangerous behavior and MUST tell her mother, at the very least warn her that you plan to betray her trust, lest she is blindsided in such a manner that might encourage her use such favorites as "backstabber" when describing the highlights of your many attributes.
And finally, I appreciate that you think you are "friends" with his mother. But you really aren't. Because only equals can be friends. She may be a role model. She may be a trusted adult. She may be someone who enjoys your company on a deep level, as you enjoy hers. But you are not equals. She is twice your age, with twice the experience. And she is the mother of your friend. And when she expresses that she is glad your friend is not sexually active, she is not sharing this information as one mother might to another. She is sharing at a mother to YOUR friend. Which is to say, in her own delicate way, she is checking her facts. because if she knows you tow are friends and has a shred of intelligence she is aware you have the inside track on what is really going on in her daughters life. And if she has two shreds she is aware of the conflict this might present and feels no qualms taking advantage of it.
She is not your friend. She is an older non mother figure you can relate to. And that is wonderful.
But these are not two autonmous and equal friendships you are weighing against eachother. And as such it is on you to treat your "equal" with the respect she is treating you with, by investing her trust in you, and at the very least, your role to change the subject when her mom brings up her sex life without her in the room. Or, at the very least mention to the mom that you aren't comfortable discussing your other friend's sex life when they aren't around. Because that is what friends do.
And finally, I would encourage you to do another thing friends do. Open up a clear, honest dialogue with your friend about protection. You mention that you are not sure if she is using contraceptionm, and that is one of your motivations for wanting to speak to her mother. Have you asked her? As a person she has put her trust in, you are in a unique position to affect her behavior. She has not entrusted her mother with this information and that does not mean she necessarily trusts her opinion enough to follow her good advice. But she seems to trust yours. Before you open up a potentially caustic dialoque between her and her mother that she has not given you permission to open, I encourage you to further open the dialogue between the two of you. Which is to say: this is a unique situation to build trust and establish boundaries among the people you are close to, as opposed to betray trust and potentiall ruin several friendships for years to come.
A concerned citizen who thinks Amy must be part of some creepy fundamentalist parenting group to give such advice.