What should a girl do who took a book from a foreign school and then moved without returning it, and she is now in another country? Should she give charity on their behalf when they are disbelievers?

You must return the book you borrowed to the school by any possible means. If that is not possible after making an effort, then you must give charity on their behalf. It is obligatory to search for the rightful owner to return their right to them. If they cannot be found, then give charity with the intention on their behalf. If they come one day asking for their right, inform them of what you did. If they approve, then so be it; otherwise, pay them their right. Charity benefits disbelievers in this world, as the Prophet (peace and blessings be upon him) said: "Indeed, Allah does not wrong a believer concerning a good deed; he is given for it in this world, and he is rewarded for it in the Hereafter. As for the disbeliever, he is fed with his good deeds for what he did for Allah in this world, until when he reaches the Hereafter, he will have no good deed for which he will be rewarded."