The text for the attachment O Lorien reads:

"Song.
Attach to hero.
Action: Exhaust O Lórien! to lower the cost of the next Silvan ally played this phase by 1 (to a minimum of 1)."

It does not state played by you or controlled by you and because you clearly choose the planning phase it seems like you can allow another player to play a Silvan ally during their turn in the planning phase and reduce its cost by 1 when playing with two or more players or decks. Thoughts?

No thoughts. Simply play it as it reads, there is no hidden meaning

Yes sure is affect any player…..

Yup, it works for other players. That's one of the reason several people, including myself, are creating 2 Silvan decks and playing them 2-handed.

Nice! Have few silvans in my deck and it is nice that silvan partner of mine can help save some resouces in case he do not summon anyone, or in scenarios like escape from dol guldur where we are limited to 1 ally per turn.