Not sure if you want to actively try to help your friend or not.
If you do, it seems like one approach would be to split the problem into two separate points:
Point 1: A neutral discussion with her explaining how when selecting cast members, your responsibility is to the production only, and your goal is to have the best person for the job. Period. Friendships should have no bearing on this. The fact of the matter is that there are loads of highly qualified folks trying for the same spot, and you would be doing a disservice to the production if you selected anyone but the best. And this should not reflect on her quality in any way—she should be able to observe other highly qualified folks who were not chosen.
Point 2: A frank discussion, completely divorced from the casting process, about her professional demeanor. Clear statements about how behavior is perceived by others, as well as firm recommendations as to what should never appear on FB. This is the kind of thing that probably will just piss her off, but maybe she might reflect on it later on and grow from it.
Both of these points should be made in private.
That’s how I would approach it.