The Shadow Druids should be an arc all the way to Act 2 and 3. Halsin's companion quest should be about the Shadow Druids and Hag Coven.
Anyways, the way to deal with them is to find her hidden letter in her chest, go kill the woads in the swamp, loot the hidden evidence in the swamp and bring it back to Kagha. The Shadow Druids will reveal themselves. You then have the option to convince her to see she's gone the wrong path, and she will join you in battle against them, and if not she will fight alongside them.
Halsin has nothing to do with the Shadow Druids in Act 1 unfortunately. He returns, if you inform him about the Shadow Druids he is furious at Kagha and demotes her, and she awaits trial by the druids while she starts anew as a low tier acolyte. But the game never tells you what happens at the grove afterwards, Halsin says he appoints some Francesca to bring in fresh blood and perspective. He even says she's handling things well in Act 2, but doesn't really go into specifics and unfortunately much of the characters in Act 1 are forgotten there by Act 3.
It would be nice to see that part in act 3, especially since Halsin has zero personal content in that part of the game, I agree with you, but if not, I hope at least one expansion. In fact, I'm keeping the wicked hope that they'll add some content to Halsin in addition to other improvements.
