Rhubarb leaves are poisonous because of the high concentration of oxalis acid in the leaves. It can cause vomiting and other issues. If too much is compnsumed it can cause severe illness and kidney problems.
Do you have activated charcoal?
Activated charcoal can scrub up any toxins that may still be in her gi tract (I know it’s been several hours since you posted)
Apple cider vinegar, watermelon, any very watery fruits can help flush her kidneys and overall system too.