Is her comb purple all the time, or just when she's at rest on her keel? In other words, when she gets up and moves around, does it return to red? Even if it doesn't, she probably has congestive heart failure. But if it returns to red when she gets up, she's not as advanced in the heart failure. She probably won't last much longer, I'd suggest culling her now, both so you at least get the meat, and to end her suffering. CHF doesn't feel good, for man, bird, or beast. Is her breathing starting to sound wheezy or bubbly?
If you cull her, and dress her out, (the meat's fine, it's not an infectious disease) you'll see a lot of internal fat that was pressing on heart and lungs, and an enlarged heart.