You have a couple of options. Work at different pts of the range of motion. She can work a flexed arm hang for time. Help her in to the top postion and let go, hopefully she'll hang there for a couple of seconds and then lower herself or push a chair back under her.
She can also work on weighted or non-weighted negitives. Help her into the top postion, use a chair or something. Pull the chair out and she can lower herslef slowly.
Start slow with only a couple of reps and sets per workout. The arm hangs she could do daily if she isnt sore.
Another option is the use jumpstretch bands for assistance, wrap the band around the bar and then she canplace her knees in the loops and it will pull a proportion of her weight up.
For the most part, pulldowns will not improve your pull ups. They use different grooves and are 180 degrees out of phase with one another. Open versus closed kinetic chain. Concentrate on the above and tell he to be patient.