Neck pulling during play

During play, my 4 year old Great Dane constantly puts his mouth around my friend's dog's neck. Although he isn't clamping down, my friend's dog sometimes whimpers because he pulls her fur (she's a golden). He doesn't do this just a few times during play, he has his mouth around her neck for almost the entire play session, to the point that her neck ends up wet with lots of saliva.

She never tries to hide or run away, but she also never does it back to him. He rarely does this with male dogs. I've tried everything from immediately removing him and walking away to give him a minute or two to calm down, to keeping him on a long lead and pulling him back as soon as it happens, to giving a firm "off" or "no" command, which he is usually responsive to in other situations. I also reward appropriate play.

I have a big backyard, and even when they're running in a straight line, he runs by her side her with her neck in his mouth. He's intensely focused on her neck at all times. I understand this can be normal, but given that he has caused her discomfort and her owner is uncomfortable with it, I'm trying to figure out how to stop it. It's exhausting constantly acting as a referee and hovering over them to break it up every 30 seconds.