I wasn't sure if only the player who controlled the hero that POL was attached to could discard cards to trigger the ability or if anybody could. Or if it has to be the player who actually played POL initially.

Example : I am playing Lore and my wife is playing Spirit. I play POL and attach it to her Eowyn card. Who then can discard cards to trigger the bonus? Me, her, or either one of us?

Protector of Lorien's ability can only be triggered by its controller. From p. 23 of the rule book:

"Actions are always optional, and can be triggered by their controller during any action window in the game sequence." [emphasis mine]

Note that Eowyn, who has a somewhat similar effect, expressly states that any player may trigger her ability. In general, though, you must control a card to use its Action: effects.

In your example, if you attach PoL to her hero, only your wife can trigger its ability (since you automatically give her control of the attachment when you play it on one of her heroes).

Thanks!! I thought that was probably the case, but I wanted to run it by the folks here just to make sure.