I'm not a musician but have had a bit more interest in the industry than the average person, so I'll give my crazy ideas on the off chance it might be helpful.

Both Bandcamp and CD Baby seem popular with independent musicians as a way to make some money selling music, although I'm not sure if anyone makes a living from them. They can at least provide non trivial income in some cases. One thing I like about Bandcamp is when musicians offer a discounted "buy everything from this musician" option and at least personally I will spend more money more easily when that is an affordable option vs. needing to try to find music I like best to actually purchase. Soundcloud is popular for discovery. I'm sure your friends are already some of the amazing musicians on these platforms that somehow can't get a wider audience for no obvious reason.

I didn't like this article that much, but I think one thing that seems right to me is how important film is in turning excellent musicians into popular musicians. Computer games should be the same, but for some reason the industry accepts a half dozen 30 second loops as being acceptable music for games that take tens of hours to play.

Hopefully this situation will improve, but meanwhile there are occiasional games that have the explicit goal of promoting music and a few that just happen to use better music. Three I can think of are Beat Buddy, Symphony, and Braid. The creaters of Beat Buddy explicitly had the goal of promoting music and I'm fairly sure they worked directly with the musicians. I'm fairly sure Symphony also had music promotion as a goal but I'm not sure if they worked directly with the musicians or not. Braid just used music from Magna Tune, but the musicians got exposure from the game. I'm not sure if any of these cases necessarily translated to income, although I suspect it did in some cases at least (maybe you could ask some of them or see if they have talked about it publicly if you wanted to look into this further).

There is also a risk in working with game developers, since the music industry likes to categorize musicians in strange but limiting ways and almost no one wants to be categorized as a game musician (same, to a somewhat lesser extent, with film). Also, the route to gaining popularity via games would likely involve indie games and it can be hard to figure out if a particular developer a) will actually be able to produce a game that meets even the low technical standards of the game industry, b) that the game is fun in some way, and c) that more than a tiny number of people actually buy the game. Even better funded game projects often don't work out for various reasons. So a lot of risk but also a lot of opportunity since good music (not typical game music) can really improve the experince of a game. Unfortunately, both game and film producers seem to often want music to be bland and annoying so there is an extra challenge in finding someone who appreciates the value good music can add to a project. To be fair, sometimes other audio need to take priority, particularly in film, but IMO most games would do better making good music the focus of the audio experience. In film there as at least usually the credits music for arbitrary music that matches the mood of the film.

I think with indie film makers it would likely be easier to evalute the ability of the producer to complete the project, although still no guarantee that many people will ever see and hear the result. If working with independent film or game makers they will not be able to pay much of anything up front, so it would potentially still be a lot of work for a tiny chance of more exposure, although both films and games often get music from a number of sources which can lower the amount of work for each musician involved. To the extent that they are already doing a lot of work for a tiny chance of more exposure it might be something to consider, making choices with some idea of where they would like their income to come from in the future. I'm not sure how a musician would go about trying to find such a collaboration, but I would guess there are online forums where it might be possible.

For that matter there are professional musician forums; I don't know particular ones to recommend, but if your friends haven't looked at them yet they might find better advice there.