Here are some studies to consider using in any discipleship relationship:

Gospel Centered Life by World Harvest Mission.  An excellent 9-lesson small group study intended to help participants understand how the gospel shapes every aspect of life. Each lesson is self-contained, featuring clear teaching from scripture, and requires no extra work outside of the group setting.

Here are some great books related to the topics of discipleship and evangelism:

Gospel-Centered Discipleship by Jonathan Dodson. Very readable encouragement discussing how to do discipleship well while keeping it centered around the gospel.

Make, Mature, Multiply: Becoming Fully-Formed Disciples of Jesus ed. by Brandon Smith. A great collection of articles on all things discipleship compiled by the folks over at GCD.

The Master Plan of Evangelism by Robert Coleman. A classic from a man who has lived discipleship well.

One to One Bible Reading: A Simple Guide for Every Christian by David Helm. Super-simple encouragement describing how to sit down across the table from someone who does not believe and read the Bible with them.