Is the money earned by my sister from her work as a secretary in a Spanish company, which involves her frequent travel with her boss and her absence from home for days with the parents' approval, unlawful (haram)? And are the gifts her boss gives her unlawful (haram)?

Your sister's travel and private meetings with her boss at work are clear prohibitions and major sins, based on the Prophet's (peace be upon him) sayings: "Beware of entering upon women," and "No man should be alone with a woman unless she has a mahram (definition) with her," and "No man is alone with a woman but the third of them is Satan." Scholars have unanimously agreed on the prohibition of a woman being alone with a non-mahram man, and the prohibition of a woman traveling without a mahram. Therefore, your sister's work as a secretary for this man, with these dangers, is unlawful, and the salary and gifts resulting from it are unlawful. She must repent to Allah and leave this work immediately, and her parents' permission does not make these prohibitions permissible.