Given they're currently spirited away to the First, and we're going there specifically to go find them, why would they be in the woods if it on the Source? They're stuck in the First until either late into 5.0, or some point in the 5.x series... Y'shtola referring to a "friend" could be referencing any number of things; Waiting for Thancred to return, or the twins. Us, as we're busy doing the Greatwoods scripted dungeon, or even a return to the Source for whatever reason. Heck, they may have made a new friends in the First... I can see how the line could be interpreted as "until our friend returns [from the First]", but that's definitely not the only way to interpret it...
The Greatwood is basically 1.x Black Shroud; Trees so grand they're blocking out the sky (main argument for it being the Firsts version of the Shroud I'd say). I'd say that's the main reason it doesn't seem to be being taken over by Light. And who's to say the people of Novrandt are the ones bombing it? I'd say they're probably taking refuge from the endless day there, and Sin Eaters probably don't take kindly to that and start bombing the place... No forest canopy, no protection from the Light, makes sense to me.
Honestly, given the other reason we're going to the First (to stop the Flood of Light), it makes much more sense to me for Y'shtolas Captain Kirrahe line to refer to holding back the Flood while we're away, than anything else...
Lastly, if this screenshot is from the Greatwood (could be a different tree'd zone though), then the sky we can see is very First-y;
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EDIT: Oh, I'd completely forgotten about the new dungeon we were shown;
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That's probably almost certainly related to the burning of the Greatwood we see in the trailer, likely the dungeon that leads up to us accessing the area even, given it seems to be on the edge of the woods and the zone itself already shows the scars of what we see in the trailer. I'd say that's more than certainly related to "Until our friend returns, I will hold the line".
Regardless, I'm almost immediately reminded of Final Fantasy V with all this Greatwood burning... I'm calling this dungeon "Village of Moore" until proven wrong...