If you used objective, business driven criteria for making your layoff decisions, then you should have no concerns. If you used the salad bar approach and laid off a little of this, a little of that, so that there is no formula that you can map to what you did, then you should rethink things.
In either case, you may want to consider a release and settlement agreement to go with the layoff in her case. In my opinion, you should do that immediately upon giving notice of the layoff and not wait to see if she balks, but that's a matter of style and your decision can and should be affected by what you know about the context.