I used to be that way. Lol
Make her understand that kicking a lazy horse doesn't hurt him at all. Horses kick each other ten times harder than a human can, so there's no way she's going to hurt her horse. If the horse doesn't want to move or is going too slow when she gently squeezes, it's time to be more assertive.
Maybe she should try blunt spurs or a dressage whip. If she winces at the idea of a whip (like I used to) remind her that it's only an extension of her arm and is not to be used to hurt the horse. At the halt, ask the horse to walk with just a slight squeeze at first, and if he doesn't respond or ambles off, tap him sharply behind your leg with the whip. Don't whack him, but just tap hard enough to wake him up. If he gives a response -even a startled one- by moving up, bring him back to the halt and ask for the walk with a slight squeeze with the leg again. He should move forward with 100% effort, but if not, repeat with the whip until he gets it. The goal is to whisper your aid and have him shout his response. This method worked wonders for my lazy draft horse. :) And lesson horses tend to be hard-sided because they're used to children bumping them all the time. Has she had a bad experience with horses before? If not, she shouldn't worry about a horse bolting. Especially if she's riding a seasoned lesson horse. Plus, of you do the exercise I mentioned, she shouldn't have to kick him at all.
Worst case scenario, you could do what my trainer used to do and threaten to put her back on the lunge line if she doesn't get over it.