Difficult employee advice

So there is a girl at my work, she’s very nice but she’s extremely negative about everything. She constantly makes everything about her and continues to bring down morale. Along with that she hasn’t even been around for 6 months and she has taken more days off than 2 of our oldest employees combined. She just isn’t dependable. She does have some medical condition so she is given leeway but the attitude really needs to change. Upon kindly addressing it with her, she started talking about discrimination. Now everyone is walking on eggshells with her and we don’t know what to do. We live in an “at will” state if that makes a difference but fear any false accusations.