Upload a video in someone else voice

Your friend could be a "co-instructor", where he or she narrates the course and you develop the script. Just be sure to read and follow Udemy's policies at https://support.udemy.com/hc/en-us/articles/229605728-Co-instructor-Relationships-Rules-and-Guidelines

Be sure you have clear, written expectations about who will do what by when, and how you will split the revenue from the course. Otherwise your friendship might not last!