Is taking some items from the plane, such as blankets and cups, considered stealing, and what should be done to atone for that if it is?

You can find out if items included in the ticket price are allowed to be taken after the trip by asking the specialists at airports or airline companies. If you confirm her right to take them, then there is no objection. However, if she learns after asking that she does not have this right, she must seek forgiveness and return what she took to the company or its value. If returning it is not possible and the company does not absolve her, then she must donate its value to charitable causes.