Is it permissible to seek the help of Muslim jinn to find out information about a specific person? And what is the expiation for that if it is forbidden? And is what my friend does, seeking the help of Muslim jinn to assist those afflicted by possession and sorcery, permissible or forbidden?

Prying into people's private matters is forbidden by religious law and naturally abhorrent, due to the harm it causes to people's lives and the disgrace it brings upon them. There is no distinction in this prohibition between seeking out such matters through humans or jinn. Therefore, it is not permissible to investigate hidden aspects of the life of the person you wish to propose to, whether through humans or jinn. However, it is permissible to inquire about apparent matters through customary means. Jinn have access to what humans cannot see, but they are not trustworthy sources of information and they lie, so one cannot be certain of the veracity of what they report.