Brought home a severely underweight BP baby. Please help me get her back on track

[Spoons]

So I went to a big box pet store today. And, like I always do, I looked at the BPs. There were seven of them in the tank, and all looked okay except one. She was lying on the Aspen in the middle of the tank. Her whole body is deflated, she's wrinkly with sagging skin. I grabbed an employee and the manager and pointed it out, and they explained the feeding protocol and what they were doing about it (not much.) I was horrified to hear they feed the BPs pinkies. Stuck shed, too.

I have always been firmly on the side of "don't buy from big box stores" because it just feeds the practice. But when the employee took her out, and she barely even moved and just kind of hung mostly limp, my heart shattered. I asked if, given her condition, they would discount her price. They said I could take her for 50% off, and on top of that 50% off of everything I bought if I also needed supplies. It sounds like the non manager guy has snakes of his own and knows she's in bad shape but per store protocols can't do much about it, and really wants her in a good home. The 50% off the rest of his purchase was his idea, not his bosses.

I took the offer. They brought her paperwork, and it looks like she hasn't eaten in 7 weeks save for one pinkie last Thursday. She's lost 11 grams since arriving (at 12 they take them to the vet for force feeding, but the non-manager employee said last time they did that the snake died there.) She's set up with everything but a heat pad, waiting on a thermostat for that.

My question is when should I try feeding? She's so thin I want to get her to eat as fast as I can, but she's just been moved so I know she needs a few days to settle. What size? I am horribly squeamish about feeding live but if I need To I will. I wanted to try a fuzzy mouse first, with the hopes that with proper humidity she will take it, but should I offer live right off the bat? She HAS eaten frozen, just not recently, I'd guessed because of horrid husbandry. I'm not sure what I should do for her to get her back on track.

Help? Here's a pic of the poor girl.

Edit: idk if the pic is showing, so here's a direct link: https://imgur.com/a/x1xPRBH