Not sure what this band is hoping to achieve. Most of the lawsuits for the WAC Bennett dam were settled a long time ago. This is the only part I see as a valid claim and they have to show they were never consulted for it and that they never negotiated a settlement in the past.
As for the Site C dam, way to early to come to any conclusion. I wrote Terms and Conditions into the approval to review data from downstream Water Survey Stations at 5 and 10 years of operation and compare that to the predicted modelling. We are not yet at the 5 year mark for initial review.
How a dam in BC affect the Peace Athabasca Delta (PAD) is that occasionally (about every 5 years) the Peace river would create an ice jam at the confluence, causing the Athabasca river to back flood the PAD and recharge it's water levels, Weather and water flow conditions on both systems would have to be just right for it to happen. Since the WAC Bennett dam went in, it changed the frequency of the event to around 10 years if at all. Compounding the issue is that water withdrawal from the Athabasca River upstream of the PAD also affect the backflooding or lack thereof.
The government many years ago built some small dams to help create backflooding events, with some success, but not enough. Some expert argue the PAD was already in the process of drying out, before the dame. Needless to say, those ones get hired by the companies and the ones that disagree get hired by the bands. As I recall we did consult with this band and their concerns was one reason we put in the data review conditions.
This lawsuit might be a spoiler for another proposed "run of the river" dam on the Peace.