K.W.
It sounds like you see this family often enough. Do NOT post anything on Facebook, or even send a private message on FB. She will just delete you and I think that will make things even worse. Instead, the next time you see her (don't mention the FB status), simply ask how she's doing and let the conversation go from there. She was probably just venting.
Now, if this FB post can potentially cause problems as far as custody of the teen goes, then you may want to take a different route. Just tell her that you've watched her grow and know how frustrating it is to be in her position, everyone gets angry with their parents/caregiver, but she's in the best place for her. Venting on FB could actually be used against her grandparents as a weapon, and that though she feels this way now, or at that time, it could cause her pain down the road.
P.S. You are a good friend for watching her online behavior for her grandparents. They are lucky to have you as a friend.