Injured Baby Garter Snake

[Mecharic]

Snake is about 5-7 inches long, brown body, 90% certain it's a garter. It was struck by a shovel while I was digging out a pond in my backyard, and is accompanied by an uninjured snake as well. Currently have it in a 10gal I never bothered to empty from ages ago (has sand, can be changed), with some water and no food.

I have no idea what I'm doing, as I've never even tried to own a pet snake before. I just feel horrible about it and don't want the poor dear to die. Photos of the injury should be attached, but frankly I haven't used a forum like this in years so if they aren't here's a basic description: Minor kink in the back, but a large severing of the scales/skin, revealing darker colored internals. I'm not sure if any organs are exposed or if the muscle is black. Snake can move it's tail a little, but cannot move properly due to the injury. This does not seem to be stopping the snake from trying though.

Is it too late for the baby?

As a side note, the uninjured one, I was planning to keep it indoors until I finish with my yardwork, so I don't hurt it later. Is that a good idea or should I put it back outside and hope it doesn't move into a dangerous area?

I am located in Southeast PA, so whatever species of garter lives in the area, that's what I've accidentally hurt.