Your friends can just request Steam keys and sell them outside of the Steam Store. Then there's no 30% cut. That cut only applies to games and software sold on the Steam store.
Valve does offer the best features on their storefront by far, thus the 30% cut. And honestly comparable storefronts (Sony also 30%, Microsoft also 30% up until August 2021, and Nintendo also 30%) all take comparable cuts. This isn't anything new or unique to Valve.