Rumil's text triggers when the FP player can spot a wounded minion. This happens after a Nazgul exerts as a cost of
Relentless Charge. Then when the effects are performed, Rumil is wounded because he's now an archer.
To turn this around, why should Rumil wait until after the effects are performed? If he has to wait, then why are response actions (e.g.
Unheeded) allowed to step in? The rules use the same "trigger" terminology for both. Response actions have a trigger describing when they may be performed. "While" text also have a trigger describing when they take effect. It seems like a stretch to say that when a minion exerts,
Unheeded can detect this trigger immediately, but Rumil cannot.