A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 25 February 2012
Shosuro Aroru (Experienced 2) Scorpion Clan • Ninja • Duelist • Experienced 2 • Unique Will not attach non-Ninja Followers. Reaction: When an action would target a Ninja at Aroru's location: Choose another of your cards. The action targets it instead, if legal. After the action's resolution, you may move Aroru to a different location. Battle: Other players may not take Reactions until this action ends. Ranged 5 Attack.
Moshi Umiko Mantis Clan • Thunder • Shugenja • Naval • Storm Rider Umiko has +2F while she has a Spell. Reaction: After a Ranged Attack is targeted: Give the Ranged Attack +2 or -2 strength.
When Aroru is trying to shoot Umiko down can she reduce his ranged attack?
Brook Cunningham (rules official), 25 February 2012
Shosuro Aroru (Experienced 2) Scorpion Clan • Ninja • Duelist • Experienced 2 • Unique Will not attach non-Ninja Followers. Reaction: When an action would target a Ninja at Aroru's location: Choose another of your cards. The action targets it instead, if legal. After the action's resolution, you may move Aroru to a different location. Battle: Other players may not take Reactions until this action ends. Ranged 5 Attack.
Moshi Umiko Mantis Clan • Thunder • Shugenja • Naval • Storm Rider Umiko has +2F while she has a Spell. Reaction: After a Ranged Attack is targeted: Give the Ranged Attack +2 or -2 strength.
When Aroru is trying to shoot Umiko down can she reduce his ranged attack?
No, she cannot.
The Ranged Attack isn't targeted until after the preceding effect, "Other players may not take Reactions...", has taken effect.