I grow ornamental plants for a living, and am also a pet owner. The best advice to give anyone who thinks their pets may have ingested a toxic plant is to call their vet immediately. Sometimes getting the stomach purged is the most effective thing you can do to prevent or lessen the consequences. This is especially important for certain plants where there is no effective antidote, and the only treatment is supportive care and good luck.

I don't freak out at all the outside plants with potential toxicity so much as I do those typically brought into the house where a house kitty is tempted to chew anything green. Some cats tend to do this, some don't. But, one of mine was a chewer and I took care to be sure bouquets and potted material was at least not dangerously toxic.

Hope the kitty improves, and this is a question best left to her vet to answer.

Yes, the vet should know about the hydrangea possibility.
However, be aware that oral metacam can have very serious side effects on a cat's kidneys and liver, especially an older cat, and a sick cat (with no diagnosis).
Hope this cat will be O.K.,
Anne-Marie