I believe I know how this interaction should work, but there was contention at both places I played about what would happen. What happens if you use any of the ready and fight with effects on a creature that is stunned?

Regardless of board state, so whether there are opposing creatures or not, the stunned creature exhausts and becomes unstunned.

3 hours ago, Tsano said:

I believe I know how this interaction should work, but there was contention at both places I played about what would happen. What happens if you use any of the ready and fight with effects on a creature that is stunned?

From page 12 of the rulebook:

" STUN, STUN STATUS CARD
When a creature becomes stunned, place a stun status card on it. The
next time the creature is used, the only thing that happens is the creature
exhausts and the stun card is removed. It does not reap or fight, and any
“Reap:,” “Fight:,” or “Action:” abilities on the creature do not resolve.
Constant abilities and abilities that do not require the creature to reap,
fight, or be used are still active.
If a stunned creature is attacked, it still deals damage to the attacking
creature during the fight.
While a creature is stunned, it cannot become stunned again."

When the creature is used is the key there. Fighting is considered a use of the creature, so in the case of "ready and fight" the creature would ready, but since it is stunned all that happens when it fights is the stun marker is removed instead.