I had the same bug today with Lae'zel. I had to reload my save, because there still is no possibility to tell her that I do not plan on helping Anders.
I think some additional dialogue options are also missing in this quest.
I met Karlach first, and decided to help her.
When I arrived at the tollhouse and first talked to Anders, I had only two options: "Help I can offer. What's going on?" and "You'll have to find it elsewhere."
I would like to be able to ask what is going on without offering help. My Tavs tend to rely more on social skills than fighting skills, so I would like to be able to gather some information first and use this to at least try to convince Anders that what he does is wrong.
Most likely, my Tav would still fail, but from a roleplaying perspective, it would be nice if she could have at least given it a try.
I have not played a paladin yet, do they have an option to appeal to Anders' conscience?
But I would also like to mention something positive concerning this quest: I think it is a big improvement that Cyrel is now walking around and even leaving the room, so that we can cast Speak with Dead without her standing literally next to the corpse.
I know that it does not make a difference - Cyrel would not have interrupted my Tav when she started to talk to her dead companion - but for roleplaying reasons, I prefer to wait till she is out of sight.