I'm not entirely sure who/what the main market is for commission painters. I would guess the biggest segment is private individuals who want well painted miniatures/armies to use in their own private games.
Another significant segment would be people like me, who futz about with selling 3d print models or minis on a mosly hobby level. Getting someone to paint up a couple of miniatures from the catalogue would be great, as they make vital promo material.
The real pros probably handle painting in-house, or via special, long-standing arrangements with select painters. So that's not a viable market.
Anyway, first step would be to specialize your service to the needs of one of the above segments. Realistically, your friend might be best served to offer army deals, where the bar is slightly lower when it comes to the quality of the paint job. Or maybe focus on some very niche style of painting, like working exclusively with one certain space marine chapter, or making faux-bronzed statue paint jobs.
Like Schroeds said earlier, the expectations are probably a little skewed. The competition is seemingly super tough, though I think this might be a bit of a mirage. There are plenty of insanely good painters offering their services, and their prices often aren't really that steep. But it seems the demand far outweighs the supply, so the good painters have a really long waiting list. A lot of potential customers can't handle the waiting, and would thus settle for less from another supplier (if only they could find one!).
Speaking as a (theoretically) potential customer, what I would like from your friend is upfront, no-nonsense info about what he offers and what the customer can expect. I you are a good, not great, painter, don't try to obfuscate this. One of the reason potential customers have unrealistic expectations (I suspect) is because suppliers all tend to try and compete at the top level instead of just saying "I don't offer services at that level. I offer services at this other, lower level". At the moment I think said lower level isn't quite established as a viable segment in people's minds, even though the demand might be there. Clearly branding yourself as a low-level painter might actually be a good move, as there aren't many (visible) competitors in that segment.
I've always found being active on hobby forums for the sake of promoting your services to be slightly precarious, so I've hitherto not bothered. I'm sure you can do it, but you have to be measured. Sounds like your friend overstepped the boundaries. If I were ever to start posting on DakkaDakka, CMON et cetera I'd probably check with the moderators in advance, and then just be 100 percent upfront with other forum members about why I'm there (to peddle wares) instead of pretending to be there for the social interaction. I think some forums have dedicated peddler's corner subforums specifically for that purpose.
If I were to hire someone to paint my minis, I would probably look for someone in my local area. It makes communication and transfer of the miniatures so much easier. So your friend might benefit from making connections locally. Besides that, building awareness of you services is a long-term engagement. Open a couple of social media accounts (Instagram, Facebook, Tik Tok etc) and post consistently and economically (I.e don't put too much effort into your posts, and don't engage too much with other users). Building up a sizeable following takes time, even if you happen to be really good at what you do (and most of us aren't). I've been posting fairly regularly on Instagram for close to a year now, and I'm still not up to more than 400 or so followers.
Set up shop on whatever platforms allows you to offer your services (Fiverr and whatnot- I don't really know what platforms are around.). You might want to cast a wide net in this regard- don't just offer commission painting, but also paint minis unsolicited and offer the painted minis for sale on E-bay and so on.
Working on photographic skill is another important factor. The obvious reason is that you want to present your paint jobs in as favourable a light as possible (super important). But another reason is that customers that are in the market for a painter might not just want a painted miniature, but also a good set of pictures of the minis. I know I would - I absolutely suck at both painting and photography. If I were to hire someone to paint one of my designs, I would ask that person "hey, could you also take a couple of nice promo shots of the mini? Few extra bucks in it for you". Seeing as I would mainly be after a nice couple of photos to promote my material on my digital platforms and social media accounts & whatnot.