Really need some guidance

My 8 month old, 16 lb. Bichpoo has been befriended by an 18 month old, 46lb. Labradoodle who is 4 or 5 times his size. He loves to take my puppy’s neck in his mouth and hold him down. My pup comes away from this “play” soaking wet and filthy and the Doodle’s owner thinks it’s cute. I do not. Today, after a particularly rough time last evening, my pup doesn’t want to eat or drink and is just laying at my feet. I didn’t see any skin breaks and he’s not acting painful, but I wonder if the two things are connected. The owner is my neighbor and we’re becoming friends, but I do not want my pup hurt. Should I get him a studded collar? Thoughts?