Cubbie, you need to CHOOSE, DECIDE--hallmarks in the process of maturing, apparently

!
-your GF told you she was a virgin, and told you al kinds of things that made you feel like "wow, I am THe man"...and you have, until now, a good relationship, right?
-her friend (?

) drunkenly told you (ok fine, so she reiterated the same when sober), your GF lied and that indeed, she slept with her ex-bf ( who happens to be the friend's bf now-sheesh).
things to ask yourself:
-why does the friend deserve YOUR loyalty? because what she told you favors you? Think about it. She lied to your GF-betrayed their friendship. So she does NOT deserve loyalty(at least, not in this sense)
-if she lied, then what will you do with the new information? Decide now, so you do not come back to LS asking..."My GF admitted to lying about her virgnity, what do I do? I love her so much, but brokenhearted about this"..
My opinion:
-Choose. Decide. If you want your relationship with your gf unmarred by some secret, talk to your GF and yes, mention the "friend". Do you want a better relationship with her or continue to protect her friend and make her believe that her friend is a "true friend" if so, then suffer in silence , or get over it or better yet, end the relationship.
However, before you confront your GF, tell the so-called "friend" you are going to have "the talk" with the GF. That way she has a running chance of concocting a story (or another story?) should your GF confront her

.
I understand that this is not about "virginity" itself but the "lying"...so make a decision and learn how to navigate the sometimes perilous voyage of being in a relationship....and caring about what other people say about your personal life.