Your fiance is wrong.
That's a 'training' collar for training. Leaving it on when it can get caught on something when you aren't around and strangle your puppy isn't a good idea. PLUS they are best fitted (everyone always buys them too big and they don't work) and used in combination with a great trainer who can tell you when/how to use it. And not until puppies are over 6 months at least anyways.
Heck I hardly even leash my pups until I have to start classes at around 6 months
Better to spend your time teaching your dog to WANT to do what they are supposed to rather then immediately using a training collar to MAKE them do it.
Have you read thru this yet? ---> Top Training Expectations for Puppies