For Those Who Are Unable To Do Even One Chin:
The biggest fallacy that I have seen regarding Chinning is that you can get better at it by doing Lat pulldowns. That is like saying that you will get better at public speaking by chewing your food! They both move the jaw bone but thats about as much as they have in common. The Lat machine will not help you become better at Chin-ups.
These are two completely different movements that should not be confused with each other. I have seen many people who are good at Lat Pulldowns but still unable to do even one Chin-up.
If you are unable to do one Chin I suggest the following: Find a Smith Machine and place the bar about eye level. Then get a bench and place it in front of the bar a few feet. Grab the bar in Chin-up or Pull-up position while placing your feet on the bench. This position should be easy enough for you to knock off 6 reps or so. If you are unable to do this, then lower the bar in the Smith Machine a few more inches and try again. You will eventually find just the right height.
I have instructed others to use a Smith Machine as one was available in their gym. However, the same thing can be accomplished with any bar that is at a lower level. This can even be done under a regulation Chin-up bar if you have something to place your feet on. You may have to get creative, but the idea is the same. You want to mimic that motion as best you can.
In addition to this I also suggest climbing up to a regulation Chin-up bar and holding yourself in the “up” position for a few seconds, and then doing a negative. Begin the first week by doing one set of three negatives, no more! Try to take at least :5 to :10 seconds to do the negative, if possible.
A third alternative, depending upon how far along that you are, is to have someone assist you from behind. As you grab the Chin-up bar and begin to pull, have your partner grab you from behind by placing his/her hands on your lower waist. Then have him/her help lift depending on how much assistance that you need. Whatever you do, do not use the Lat Pulldown machine thinking that this will help you with Chins/Pulls. It may build the Lat (not as good as Pullups) but it will not actually help you do Chins-ups or Pull-ups, as much as actually doing the movement!
Also, make sure that her bodyfat is low. The lower the bodyfat the easier to perform Chins. I know that many women usually carry more (fat)weight in their hips and thighs. This is one reason that they usually have a more difficult time performing Chins. But that is not insurmountable. Make sure that she is doing the proper cardio and following a good nutritional plan. Both will help her attain her chinning goals