The biggest problem with this question is that to write good documentation for a project you need to really understand that project. So I would honestly sugges

The biggest problem with this question is that to write good documentation for a project you need to really understand that project. So I would honestly suggest your friend takes on something open source that he/she knows really well already, and is interested in taking a deep dive into. There's not an open source project on the planet that would turn away better documentation, so it really doesn't matter what you choose. But choose something your friend CAN improve, with knowledge and understanding of how it works.