Let the witness know there is nothing improper about your prep. Tell the witness, especially a non-party, that opposing counsel is also preparing her witnesses. You will always be protected from the suggestion of improper witness coaching by telling your witness from the moment you start to prep that she needs to tell the truth.
After the witness gets more comfortable with recalling the events of the case, tell the witness you are going to ask them word-for-word an important question. Tell them your question and their answer is crucial. Then ask the question. Listen to each word. Stop the witness when a choice of words, inaccuracies or demeanor needs to be discussed. Talk to the witness about your concerns. Work on each segment of her answer. Ask the question again. Get the right answer.