Perhaps a different way of saying this is:
1. DoF is determined for this set of circumstances by f/stop and focus distance.
2. Ambient exposure is determined by f/stop and shutter speed.
3. Flash exposure is detremined by flash power, flash to sybjjct distance and f/stop.
To solve this problem:
1. I would select the Dof needed for your friend and background to both be adequately in focus, which would provide the f/stop and focus distance. There may be nothing to autofocus on at the distance you select. In that case, try manual focus. And remember, DoF also depends on other variables assumed here to be fixed (like ISO, lens focal length, print size, and viewing conditions, etc.) for a given photograph.
2. Then I would meter the ambient to determine the shutter speed for the selected f/stop. You can use ISO as a variable here.
3. Last, I would determine the flash power and flash- subject distance for the selected f/stop (and ISO).