So, it's been going around the office that one of my co-workers thinks the devil's after her. She's wound up crying, calling out "Jesus" in the bathroom, all kinds of crazy things. Everyone on our floor thinks she's insane. She told another co-worker that she couldn't handle it anymore - then she

I'm trying to figure out the best way to help her - to let her know that she's not alone, that I'm here if she needs to talk, help her get some outside help?

How well do you know her?
How old is she?
Prior to this latest info, how did she normally act/feel?
Has she experienced a stressful event in the recent past?

Based on what you described, a few conditions that come to mind:
Drug addiction
Schizophrenia
"Mental breakdown"
Bipolar Disorder

If you are friendly with her, you could just ask her if she's having any problems/stress in her life. Saying something like "I'm concerned about you, you seem to be feeling very stressed/depressed. If you need someone to talk to, I'm here for you.

Or if you're not comfortable talking to her, maybe you could have a supervisor talk to her ...similiar conversational approach, but the supervisor might be able to offer help by reducing workload, getting outside help, etc.

Or maybe you could contact her family? If she has truly lost touch with reality, you probably won't be able to reason with her. She might need to be committed to a mental hospital.

I don't really know what to do ...just throwing out a few ideas. Good luck.