When confederating Durthu as another Wood Elves faction, in my recent case Orion, but it has happend in the past with the Heralds of Ariel too, he contains his ability to use the Aspects on forest spirits that were already in his army at the time of confederation.
But if you would go ahead and put aspects on the units, and end your turn, your game crashes.
After confederation, if you save and quit your game, and then load that save again, the option to add Aspects is gone. So providing a save at this exact turn for you guys won't work as you wouldn't see the Aspects still being available. Here's a screenshot showing the problem:

You can see i'm Talsyn, turn 33, I confederated Durthu by winning 3 battles against dwarfs and hie still has the Aspects mechanic available for his army, on all the tree spirits (dryads, Tree Kin, Treeman). I can actually add the Aspects still. But I'm not gonna as I don't want my game to crash and corrupt the save (like it did in the Heralds of Ariel campaign).
No mods, I own all three games and all flc and dlc. Please make it so Durthu looses the ability to add Aspects to tree spirits in his army after confederation by another faction that doesn't have access to that mechanic!
The added save is from before the confederation with Durthu,
1)Load up the save
2) end turn
4) Use Orion to attack and settle Karak Izor
5) Use Findol to attack and settle Karak Bhufdar
3) confederate Durthu in the event
4) check the tree spirits in his army and see the Aspects mechanic is there
5) And if you dare, add Aspects to the tree spirits and end your turn (might take a couple, not sure) to see how it crashes. Which is imho logical as Talsyn shouldn't be able to add aspects.