It can be a challenge to find reading material of interest to teenage learners written at the appropriate level of English.
Love to Learn has a range of early readers, and can order in books if you need something else.
Newsround has short articles of this nature, suitable for beginner/elementary level mentees.
Simple English Wikipedia is good for slightly more advanced readers.
Check that they have access to their public libraries and know how to access books from their school library. Love to Learn can pay for reading materials if you find books that you feel are appropriate for your mentee. Novels can be broken down and made more accessible by setting reading homework each week (eg. to read a chapter), and by asking your mentee to summarise it at the beginning of each session. Shorter books like Roald Dahl’s The Twits are ideal for this.