Click on the traveler's box inside your itinerary.
Make sure that the traveler has an an email associated with their traveler profile.
Look in the bottom left corner. Click the "Resend Invite for this Itinerary" link.
If you'd like, you could also copy/paste the traveler's unique app credentials in a personal email and send them to your client. Make sure to tell them to download the app from their App Store or Google Play.