If you see this plant continuing to march downhill as you are saying.....I'd dump it even with proper ph. She may have contracted some sort of root disease or fungus and I'd get her out of your growroom if she looks that bad.

Also consider a good root product like Cannazym and/or sensizym (sp?) for your next grow to keep roots in their best condition.

Glad to hear the others are ok......sorry about the one.