Any freelance editors in the house?

Any of my peeps who have done freelance editing work (or writers who have used one) want to give advice? A colleague of mine, whose writing I have edited as part of my real job, has asked me to work on a side project editing a book he's working on (non-fiction, spiritual/religious - he's also a minister), like, for real money and everything. Which I have never done before!

I've looked at EFA's website for rates and such, but if anyone has tips on things to consider in freelancer/client relationships, please let me know! He's worked with an editor before, someone he found off a job board, but felt like she was putting too much of her own voice and writing style into it. My previous work with him has mainly been short (1-4 pages) reports, so I think I'd probably suggest doing one chapter or part of a chapter to see how it goes before committing to a whole book. I don't know exactly how much he has finished, enough to start sending me chapters, but I also would need an estimate of how much longer he expects to be working on it, and how many chapters it would be, right? So I'm not committing to a George R.R. Martin-length saga that will take 10 years to finish a single installment.