Hi, literally 'asking for a friend' as I have little expertise in this area.
Is there any member who is a proofreader, passed a proofreading course or had success (or failure) as a proofreader?
A friend is considering this as a potential new career direction. She is willing to take courses and wants to provide professional proofreading services if she passes. Is this a saturated option with little chance of success, or do publishers requires those services, taking chances on newbies?
Has your friend looked at the Chartered Institute of Editing and Proofreading (CIEP)? They're the industry body for proofreaders and have lots of helpful advice. They also provide the training courses.
I work as a proofreader, I've been in the industry over ten years. It is a saturated market place. Lots of people want this type of work. But of course there are always opportunities for new people coming in. If your friend wants to work with fiction (there are lots of other pieces of text she could work on) then it seems a lot of proofreaders get work for independent authors, more so than working directly with publishers. I work directly with publishers, but I don't work on fiction.
Your friend would need to invest a lot of time finding work at the start and I would say it could take a couple of years to build up a decent client base. If she's willing to do that, it's a rewarding career.