How can I get mom to work on her left-neglect on her own?

Interesting, I have never heard of left-side neglect. I will have to read up on that...learn something everyday here!

Most likely you won't. Even with physical therapy giving her direction and doing it for her, she will most likely not do her range of motion. Try to get her involved in her exercise by making it fun or at least interesting. And of course extend this exercise to her affected leg  & try to get her to do ROM etc on her own with direction. If not her flaccid leg  will get painful and  tight and hard to exercise. A sling can help to support her arm's weight).
Maybe get a tennis ball
& ask her squeeze it & watch her do it, with your encouragement.
Don't forget to exercise the affected leg as well or that will get contracted and hard to exercise.
Good luck!

I am sorry to say... you can't

What you need to understand is that the left side neglect is her brain. The damage to her brain has taken away the total awareness of the left side to the world. It shows in "left side neglect"..... but it really her brain that has lost any idea there there is any such concept as a left side to the world.

You can teach her tricks to help compsate...but, you will never be able to teach to compensate for the part of her brain that is dead.

I'm sorry to be negative, but I'm not at all sure you can. The neglect doing what it says, and everything, how can she be motivated to pay attention to something she can't recall to existence?

Maybe an occupational therapist might have ideas for you, though?