When do static bonuses go away relative to damage dealt?
Your friend is correct. By CR 119.6, and/or 302.7, Radiant has six damage marked on it for the rest of the turn. The damage doesn't come out of the toughness, buffed or otherwise. What happens in order is:

His spell does 6 damage to your creatures
State-based actions are checked. Your other seven creatures now have 6 damage, and are destroyed for having damage greater than or equal to their toughness. Radiant is not destroyed yet, having toughness greater than damage.
Since state-based actions destroyed creatures, they are re-checked with the new gamestate. The other flyers are no longer buffing Radiant, so she is now a 3/3 creature, still with 6 damage marked on it, so Radiant is destroyed at this point.


119.6. Damage marked on a creature remains until the cleanup step, even if that permanent stops being a creature. If the total damage marked on a creature is greater than or equal to its toughness, that creature has been dealt lethal damage and is destroyed as a state-based action (see rule 704). All damage marked on a permanent is removed when it regenerates (see rule 701.11, &quot;Regenerate&quot;) and during the cleanup step (see rule 514.2).
302.7. Damage dealt to a creature by a source with neither wither nor infect is marked on that creature (see rule 119.3). If the total damage marked on that creature is greater than or equal to its toughness, that creature has been dealt lethal damage and is destroyed as a state-based action (see rule 704). All damage marked on a creature is removed when it regenerates (see rule 701.12, &quot;Regenerate&quot;) and during the cleanup step (see rule 514.2).