Do you have to be within a certain level range to coop with a friend in private mode?

My friends barely starting his journey today and I'm already about third act in level 28, wielding the Irithyll Straight Sword. If he launches up and we search together at Lothric High Wall can we play together or is there a level restriction? I went back to High Wall to help some noobs and farm souls in public mode, but I didn't notice any soap signs popping up on my screen, which is why I ask this question.