The anger and hostility would appear to stem from insecurities which she's been unable to resolve. This incident provided her a vehicle she could use to vocalize the hate she holds in her heart. A side effect of the growing acceptance of the LGBT is the internalization of the hate that an individual feels towards that demographic. For example, racism hasn't disappeared from society because certain attitudes are no longer socially acceptable. Oddly enough, many racists don't even see themselves or actions they take as racist. They're easily spotted after hearing news stories about muggings, murders, rapes, etc because of how they respond. The quintescential question they often ask gives them away; "Were they black?". Physical chacteristics do not predispose behavior, criminal or otherwise. There are other factors in relation to behavior.
This brings us back to your friend's behavior. She's a bigot and likely felt a euphoric feeling after writing her diatribe. Later she'll probably say she didn't mean it, she was just upset, etc. She might even delete her post out of embarrassment. The question you need to ask yourself is what YOU are going to do about it. You can remain silent and allow her offensive speach to go unchallenged. Or you can privately or publically confront her and let her know you were offended.
Option one is probably the easiest for you but your silence makes you complicit with her sentiments. She'll assume those who don't challenge her agree with her sentiments in their silence. The second choice is to challenge her and let her know how you feel. Believe it or not, it's easier than it sounds. Just tell her you understand she was upset, but that her statements were offensive.
