A friend of mine tells me that an activated ability of a creature (for example, Goblin Fireslinger) can't be targeted (for example, by Reroute) if it's given shroud (for example, by Lightning Greaves).
Since a card like Reroute says "Change the target of target activated ability with a single target" and doesn't target the permanent itself, shouldn't this ability be a legal target?
If there is some ruling source about this, I'm willing to look on it.
Your friend is wrong. You can target Goblin Fireslinger's ability with Reroute even if Goblin Fireslinger has Shroud.
Activating an activated ability creates an activated ability object on the stack that is independent of the object whose ability you activated (the Goblin Fireslinger, in this case) [CR 112.7a]. The activated ability holds a reference to the Fireslinger, but it does not have any of the Fireslinger's abilities or characteristics, including Shroud.
All that aside, Shroud is an ability that specifically applies to permanents and players [CR 702.18a]. Even if an object on the stack has Shroud (such as an Algae Gharial spell), that object can still be the target of spells and abilities.
Yes, the ability is a legal target.
Shroud applies to the creature. It says you can't choose the creature for a target.
Reroute doesn't choose the creature for the target. It chooses an ability. Those abilities are not connected to the creature (once they go on the stack, they're wholly separate objects) and they do not inherit any of the creature's abilities.
