How to manage a tall plant with a thick stem in week 1 of flowering?

if you don't think you can snap it horizontally, then there's nothing you can do beside swapping out tents. I know of no magical spell that shrinks objects, but YMMV. train it in future before it does this.

What einamio said below would also be better then supercropping, unless you have experience with it.

I would use a Scrog net to try and bend the higher branches lower and get a more even canopy. I wouldn't suggest supercropping at this stage, high stress training in the flowering stage can effect yields and if you have no experience with it, it could cost you.

bend it down (carefully)

I agree with Dave on the super cropping. Only other thing I can think of is, if you have space, you can lean her over. As always prevention is better than cure and in future try to keep your canopy as level as possible before it gets this far out.

I would supercrop the branches that are too tall and try to bring them closer in height with your other plants. I needed to do this on my first grow, my northern lights diary... You will want to pinch the branches you are going to fold over to soften the branch before you begin folding. Make sure you have a plan with where you are going to fold your branches as you do not want to fold branches on top of each other. Likely will not be problem for you as you have a single plant. Lots of good resources on supercropping and growweedeasy.com. Hope that helps