I was in a sufi group before. I liked it until, well..I'll spare that for the brother's chat. Remind me, brothers if you're really interested.
I'd suggest that you just go and look around. You gotta know where your limits are. Know what is bad and what is good. Right now, I gather you're clueless as to what really happens in this circle. (sorry, I don't mean to make it so Sherlock Holmish)
If she is your best friend, you owe it to her to at least find out what she's doing.
if (that's a big IF) she has joined the wrong crowd, then witnessing the discrepancies will be benefit to you in order to refoster her understanding of Islam.
If you love her, you will try your best to rekindle her spirit for the real understanding (assuming that the sufi group that she's hanging out with not in accordance to the sunnah). She loved you enough to help you know Allah. It's time to reciprocate, don't you think?