Tell your friend to look up the Western Reaches style of play. One of the core ideas is that you begin and end each session in a safe zone so that you can have a rotating pool of adventurers. You can tuck in some rules for having mercenaries for when you have fewer than the encounters are balanced for and you’re off to the races.

It does reduce the possibility of highly on rails campaigns and instead requires more of a sandbox plan with one page dungeons and stuff. Even so, it seems made to solve this exact problem.

Our D&D group meets once to twice a month and is about seven or eight people. That's large enough that at most two of us can not show up and we're still enough to play and enjoy ourselves. The DM writes session recaps and posts them the day before the next session. The overhead here is minimized by his taking notes during the session. This has kept our group going for something like three or four years now!

The one thing about D&D is that I know almost everyone there exclusively through the campaign, and 90% of my interactions with them have been in character, which means I actually know very little about their personal lives. We're getting better with this with non-D&D hangs though.