CD Baby is what they call an "aggregator", and is essentially a wholesaler who distributes your digital music to "retailers" such as iTunes, Amazon, Youtube etc (as well as selling them directly). It's US based but operates world-wide. I signed up with them some years ago as I couldn't find one in the UK, although there are now several.

Musical rights are based on the assumption that there is a "publisher" who creates and owns the music (the musicians are mere employees). This may suit the mainstream music industry but doesn't work so well for independents. If you self-publish, you need to set yourself up as a publisher to claim all that is due to you, which if you are not a formal publishing company or record label
can be complicated. CD Baby is offering to act as your publisher and to collect performance and mechanical copyright royalties on your behalf.

Managing publishing rights is a service they offer to their members - for a fee, of course. They say "With CD Baby’s standard distribution service, your tracks are available worldwide on all the most popular digital platforms, and you earn money for the usage of those sound recordings. But without a publisher, you’re leaving publishing royalties on the table. If you create original songs, then YOU’RE the songwriter and this is YOUR money, generated for the usage of the underlying composition whenever your songs are streamed, downloaded, performed in public, and more. How do you claim it? CD Baby Pro Publishing is now available to artists in 70+ countries and territories worldwide, and we’re here to help you collect everything you’re owed — because no song should go unpublished."

So far as Performing Rights affiliation is concerned, they say "Not already registered with a performing rights organization? For United States and Canadian artists, we’ll save you the trouble and affiliate you with either ASCAP or BMI in the United States, or SOCAN in Canada. For other countries where we offer CD Baby Publishing, if you are unaffiliated, we’ll guide you to how to join your appropriate performing rights organization. If you are affiliated with a Performing Rights Organization already, CD Baby Publishing will become the administrator of your publishing. In each scenario, you will still be paid your writer share directly by the Performing Rights Organization."

All this is in the "Members" section, but if your friend is already using them they should be able to see this.

So far as I can tell, this is an add-on product they are offering, there is no obligation to take it up. However if your friend chooses to do so, this letter is the legal authority for CD Baby to act on their behalf.