I have a client who has a very aggressive shelter dog and cannot seem to give the kind of correction that is necessary to control this dog. What can I do?

I am a novice dog trainer (3yrs.) and have a client that has a dog who is very territorial and overprotective of her. Obviously, like in many other cases, the owner cannot give the dog the correction it needs to extinguish the behavior. I suggested an electric collar and she got one. My concern is this: I know that if the dog is at the height of its frenzy and gets a big correction, it will probably redirect at her. The dog cannot be muzzled at all times and I don't want them to get hurt but the dog is strong. He weighs 85 lbs. and is a freak mix from a shelter. His obedience is poor but again I cannot get them to follow through with the proper corrections when I am not around. He has redirected at me on muzzle and gets into attack mode with me at least once a session. I want this dog at least to show some improvement. Any suggestions would be appreciated. I want to be a better trainer and this case is deteriorating my confidence.

L.