Gentle Leaders have been known to 'tweak' the neck and cause a sideways whip lash effect. Gentle Leader does make aN 'Easy Walk Harness' that clips in the front and compress' the dogs chest when he pulls forward - it works on SOME dogs.
If you could help to train this dog (since you seem to care a lot and the owner seems a tad infirm?), you could help teach the dog not to pull at all and then transfer the methods to the owner.
First of all...people are the ones who teach dogs to pull. Every step you take with the dog on a tight leash teaches the dog to pull the human because it works.
Begin by standing still - lock the leash to you (leash hand with thumb in pocket works great). DO NOT MOVE ONE INCH!! Allow the dog to pull against the pressure of the leash as if he is tied to a tree. Trees are great teachers because they don't give and they don't move. The dog creates his own pressure by pulling against the leash and when it doesn't work he steps back and creates slack in the leash to get comfortable. Praise when he does this.
Do this several times in different locations until he stops pulling against the leash. You can give it a word like "close" so he learns what you are asking for.
Then you need to progress to moving. Have the dog on a short but loose leash beside you. You are going to take 2-3 steps forward on a loose leash and then stop abruptly (with a slight stomp of your foot to cue the dog). If the dog stops with you then relax, keep the leash loose and praise him. If he step 1 inch past your toe line then you are going to turn sharply into him and go 2-3 steps in the opposite direction and repeat the drill. It is very simple. If he flies past you before you even get to the 2-3 steps then you are going to INSTANTLY go backwards 5-10 steps - bringing him back to your side and when he gets beside you start walking forward again. Consider this a 5 yard penalty for rushing.
It is always best to start drills in the house on carpet. Less distractions and the carpet provides good traction.
You are teaching the dog to be aware of your body. He gets pressure for moving past your toe line by having to turn around or go backwards. He learns that pulling to what he wants only gets him further away - but walking nicely beside you on a loose leash gets him moving forward which is what he wants.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended