Answer: At this point, I'd just leave the sickly senna alone. It may resprout once the root system recovers. Since they're both flowering, the leaf loss is probably a sign of stress. Now that you've turned off the water system, the ailing plant may have a chance to recover. I'd wait until spring, when new growth begins, to assess the situation. At that time you can prune away any branches and stems that are obviously dead.
Glad you like your gardenias!
Best wishes with your landscape!
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