Overwhelmingly it’s about word of mouth.
See the reviews and recommendations on this very forum and others every day for chalets, skills instructors, cars, job advice and whatever else.
Do the basics right – especially the website (simple but up to date and with plenty of pics / video tour of the accom and area).
But IMO the product is the best marketing opp your mate has.
If your mate’s chalet is set up well, the people are friendly and helpful, and they go the extra mile, people will repeat book, they will post on here and generally champion it.
Does he ask for feedback from guests, or send out an anonymous post-stay survey, for example? There may be big clues there about how to refine the product so that the repeats and referrals start flowing.
Those are my thoughts.
When we book our first Alps trip this summer it won’t be because of banners or flyers. It will be what this community recommend…