Volumes are always going to vary as some athletes can tolerate a lot, some can’t. Some get injured easily and some are extremely durable.
I think sprinting twice a week should be fine, but just be careful. If you have to warm-up on the concrete, your sprint volume is going to be lowered even more.
To start off with, I would normally do something like 10x10-15m from a lying start. I am not sure how viable that is in your situation, but it can often be a good starting point. The volume there is really low, but is a start and you can begin to either extend the reps or increase the number. Typically, breaking it into something like 5x10m, 5x20m would be the next progression and slowly extending it (perhaps 4x10, 3x20, 3x30 would be next, etc.) as I felt necessary and useful. Vary the starting positions as much as possible
We will most likely warm up inside the facility where at least the flooring is a bit softer (rubber mats). Might have to do her drills on concrete; there is a small patch of grass next to the concrete but it’s a bit sketchy; has some holes in it (damn gophers!!).