How can one reassure a friend who imagines the jinn embodying themselves to her in the house despite her recitation of the Quran, and what can she do to be reassured?

What afflicted your friend is the magic of illusion, just as the Prophet, peace and blessings be upon him, was enchanted. The cure for this is to turn to Allah Almighty and supplicate, and to frequently remember Allah and recite the Qur’an with contemplation, for it is a healing for what is in the chests. In addition, she should recite the supplication: "O Allah, I am Your servant, son of Your servant..." mentioned in the noble Hadith. If the symptoms persist, she can present herself to a trustworthy ruqyah practitioner (one who performs Islamic exorcism). One must beware of charlatans and imposters. What she is experiencing might also be envy or whispers (waswas), and not necessarily magic.