If you haven't read Combatting Cult Mind Control, by Steven Hassan, by all means get and read through that first. One of things he mentions in his book is that sometimes people with good intentions end up coming on too strong. I know I've done (do) that and I'm learning more effective ways of helping people who are trapped by a cult.
The first and most important thing that he mentions is to build trust. Now you have an advantage in that since this person already respects you, but you also know how powerful the mind control is when a jw is told someone is an apostate. It closes things up completely. So, you want to avoid playing all your cards at one time. Let your friend do most of the talking and pay attention to what they say. If you are alert, the way to proceed may be obvious from what things are important to her. Plus, since you studied with her, can you remember what her interests were before she became a witness? Another suggestion that Steven Hassan makes is to help a cult member bring up memories of activities they enjoyed before becoming involved. That can break some of the power the mind control has over the person. Remember, they didn't join in a day and they aren't going to come to their senses in just one day either. Keep the overall goal of helping them recover their mind in front of you and work towards that.
I can't recommend this book enough.