Yup, it's gone for good, so she won't be learning that magic. What a waste, especially since I expect that the tome was likely sold at far less than its actual value (the kids don't know its value, are selling it off to someone who has to bear the burden of reselling stolen goods and then find someone who actually has a use for it and the money to buy it).
Considering that the boss already had extra money he could give her, he easily could have prevented this situation in the first place by paying for the treatment. But well, rereading the previous chapter, the kid had tried asking for help and his mother had apparently refused so he was left having to find the money himself, the only way he knows - theft.
Instead, she'll be getting a staff.