The only extended slide release I've seen for the Glock (I have one on my Glock 17) does not move the release forward or back, but simply makes it stick out farther, so that its easier to engage. Given the frame design, even if you could find one that reposition the slide release more the rear, you'd probably have to reshape the polymer.
I don't an extended slide release will help your lady's problem.
And from a more basic point of view, its not really that big an issue: she can release the slide by pulling back on it slightly with her off hand -- which is probably a better skill to develop, as the main time to be doing this is during rapid mag exchanges.
(Most defensive gun courses tend to emphasize NOT using the slide release in combat situations. And if its not combat, time isn't a problem.)
[This message has been edited by Walt Sherrill (edited April 13, 2000).]