I've searched through FAQ and rules and I haven't found anything yet, that would solve this.
Instead I found card with similar effect.
Ride to Ruin (THoEM78)
Action: Discard a Rohan ally to choose a location. Place 3 progress tokens on that location.
While I was looking through event cards all other effects written on text box pointed exactly what should be discarded or from where it should be discarded.
If this type of effect affect player hand, there are more cards that could be used that way.
In this case cards mentioned below should be put in player area than other copy in player hand could be discarded when Action was triggerd.
Westfold Outrider (TVoI6)
Action: Discard Westfold Outrider to choose an enemy not engaged with you. Engage the
Westfold Horse-Breaker (THfG6)
Action: Discard Westfold Horse-Breaker to choose and ready a hero.
In this case Westfold Horse-Breaker who is in play discards Westfold Horse-Breaker that is in player hand and effect is triggered.
Edited by c08, 08 June 2014 - 01:53 PM.