It isn't heat stress. Not at the temperatures he's saying he's had, and the fact the the last heat spell was three weeks ago. And the fact that it hardly looks like heat stress at all. It's nutrient toxicity.
OP, after you flush it. Pick it up with one hand and feel how heavy the container is. Don't water it until it feels very light. Another test is to stick your finger in the soil. If it's dry up to your first knuckle, then give it some water. You'll get the hang of it. The plant might not make it, but you'll do way better next time. Remember: less is often more. The plant is programmed by nature to survive with light, soil, and water. A little nutrients go a long way when you're starting out.
After you flush it really good, I'd give it three or four days before I'd even think about watering it. Just let it have the lights and some nice air to breathe and let it recover. It's in shock right now and time is the only cure for that.
Expect some of the current leaves to continue dying off, but stay positive and hope for some nice new healthy growth. If I were you, I'd hold off two weeks or so before throwing them into 12/12, if they make it. They'll need time to recover and get balanced out. Good luck and don't lose hope. You might have overloved your girl a little bit, but you'll learn from it, and fingers crossed you'll even get to say "I can't believe it survived all that punishment" when you're harvesting a few nice buds off of it in a few months.
we ll see .