What can I do to help?

Making yourself physically available and receptive is a great start. Whether being the passenger while the learner is driving or whether you are explaining what you are doing as you drive, how you are doing it and why you are. You may not realise it but what you did before they got their licence and what you do now forms their understanding, so explain, fill those gaps in how and why things work. Approach answers to questions with a positive and encouraging attitude, many new drivers will have all kinds of idea (not all of them correct) why and how things work.