A gait belt, which is a heavy-duty cotton belt, can give you a better hold on your mother when walking with her. Be sure to consult with her doctor to find out the primary cause of her listing to the left. A variety of things could be throwing her off balance, and knowing the cause will help you determine the best treatment.
One possibility is that your mother's proprioception -- the sensory information her brain receives from her body regarding its position in space -- is impaired. This sensory information allows her to know, for example, whether her knee is flexed or extended. Her physician or physical therapist can test which specific area of her body may be impaired.
Memory impairments can also affect sequencing movements. For instance, she may have forgotten what to do with her right leg after putting her left leg on the floor, or how to shift her weight to bring one leg forward when walking.
These impairments can affect her protective, or balance, reactions, which are reflexive responses -- for instance, extending your arm when pushed to one side to prevent yourself from falling. When someone with dementia starts to list -- say, to the left (like your mother) -- she may be having a slowed or impaired brain response telling her to correct her balance and lean to the right.
When you're using a gait belt, if she leans toward the left, stay on her right side, holding on to the gait belt and walking a little behind her. You can also verbally cue her while she's standing still ("lean onto your right foot") and give her a tactile cue by holding her gently on the shoulder with one hand, grasping the gait belt with the other hand, and pulling her gently to the right.
To increase her sensory awareness of her weight distribution while she walks, have her try "weight shifting" when she's sitting and when she stands up prior to walking. To do this, have her lean to her right foot and then shift her weight to her left foot. She should go back and forth several times, which may give her a sense of how it should feel when she's walking. A physical therapist can work with you, demonstrating how to use a gait belt and how to walk with your mother to compensate for the listing. He can also show you more activities to help your mother sequence her movements better.