My best friend got grounded, she's going behind her parent's back to hang out with friends while they're on vacation. She told me she "doesn't care anymore," ...I get she's mad, but I gotta do something about it! Am I overstepping my boundaries?

All you can do is tell your friend you think she's making a mistake acting like that.  But you're not her parents.  They need to handle this.  You'll just become the "bad guy" if you get too involved in her problems. 

Yes, you are  - UNLESS she is engaging in dangerous/unhealthy activity such as drinking or doing drugs, it is not your place to enforce her parents' punishment.

(IMO, if the parents' expect her to abide the punishment while they are gone, they are incredibly naive.)

You can remind her of the possible consequences of her actions---including the potential trust issue---but anything coercive in nature would in fact be overstepping your boundaries (this is not an "emergency") and crossing hers.

You could say, "Well, Sarah, you know what's going to happen when your parents find out that you're sneaking out to see him....." and then let it go. Not your monkeys, not your circus. She can't grow up and mature if you're there to catch all of her "mistakes".  Sometimes people just have to be caught and go through the pain themselves before they learn a lesson. This might be one of those times.