Yuuup. I got the quest, freed Halsin, then tried to go and read the letter.
I can still (strangely since the quest somewhat resolved) read the letter and send it to my camp, but no one reacts to the information.
Yeah, we should be able to tell Halsin at any point, hopefully they will fix. Otherwise Kagha admits she's working with the dark druids and implies your stupidity for confronting her about it. Halsin just gives her another chance so he probably isn't aware, and since he decides to come with you it's likely she will be up to no good in his absence.