Re: Can an editor advertise their services here?

Presumably. Personally, I think it might be a bad idea for a writer here to hire an editor to post their work here. Going so far as to say I would definitely try to talk them out of it, it's pricy and the return for posting their stories for free isn't worth it. A lotta folks on this site would be happy to give it a valid critique for free. If they wanted to upload to amazon that would be a different story though. 

That said, I'd tell your friend that it'd be a good idea to really give a thorough overview of their credentials so the administration knows they're legit. I know that there are some sketchy freelance editors out there, and most writers should be pretty wary of being taken advantage of.