She is dehydrated, but she is drinking excessively - the water is going somewhere.
Does she have acites\water belly? Does her abdomen feel like a water balloon? My hen had issues staying warm when her condition worsened and she absolutely had acites.
Unfortunately, she is an old girl and is in decline. There's likely much more going on here than what's visible and I would be hesitant to cause her any undue stress at this stage. This could a be tumor-related illness, which isn't something that can be treated.
Palliative care until she can get to the vet, or see if they can take her into ICU early if you feel she's that far in decline. The feet are a symptom, but probably not the underlying issue here.