the lesser demons are in a spirit form, they have no physical body which would be immortal. the higher ranked demons (gods) have physical bodies that are immortal. i dont know if its possible, i guess it could be...
They say it is possable demons do roam the earth as we speak in disguise. But did you friend tell you or a demon? Cause as before there are no mediators in Satanism. If this was true a demon would tell you itself.