SiteC is the worst possible decision the NDP has to make

Damned if they do.
If the NDP proceed with SiteC, they will have invalidated many of the arguments they hit the BC Liberals with.
The dam is not a $7 billion dollar project, it's likely to exceed $12 billion if allowed to continue.
This $12 billion is on top of the billions more debt the provincial government has forced upon BC Hydro while simultaneously taking billions in "dividend" payments* from the crown utility to prop up their imaginary budget surplus. Did I mention that the BC Liberals had forced BC Hydro into deferring debt payments while this dangerous liability was growing in the background.
This is the state of things now, and if the NDP continues SiteC, this becomes their mess as they will have signed on to the plan.
In doing so, the NDP risks alienating a large section of supporters who believed their promise that a negative verdict from a BC Utilities Commission report would trigger it's cancellation. Folks who generally see themselves somewhere on the Green/NDP threshold would bolt for greener pastures.
If the NDP continue SiteC because they're calculating that moderates split between NDP and BC Liberals would come aboard the NDP bus, ask yourself where these folks were when the BC Liberals destroyed 30k forestry sector jobs? My point? If they didn't flip to the NDP already, they're not going to now.

Damned if they don't
If the NDP cancel SiteC, this is generally in line with what most folks understood about the NDP platform. Though it was never explicitly published as so, communications on SiteC made it clear that a negative review by the BCUC would doom the dam. This is what I understood, that's what most people understood. Given the frosty reaction from dam proponents against the BCUC report, they understand it too.
This choice has risks too. There's no guarantee that cancellation will unite the NDP/Greens, it may stem any defections. It will not however, bring any BC Liberals on board.
But cancellation does grant the NDP the narrative that says that whole BC Hydro financing model is flawed. This is important if they're to shine a critical eye on the explosive debt, flawed IPP contracts and the like.
The NDP will likely be accused of chasing investment away, which is strange given that these are taxpayer dollars and not private investment dollars funding the dam.
The NDP will be accused of being hostile to climate change reduction by cancellation, but what does it tell you when the NDP's junior partner in government (Andrew Weaver and the Green Party) oppose the dam too?
The NDP will be accused of being anti infrastructure. False, but this will be said anyways.
The NDP will be accused of being anti-worker. If the government cancels SiteC, front line workers have a fair claim to compensation in my opinion. To be sure, remedial work will also be a multi billion dollar expense and should be done by BC workers. Further, there are many more projects in BC that can be done by these well trained and well paid BC workers.

Bottom line
Cancelling or continuing the dam will not be the end of civilization as we know it.
To continue the project means to buy into the BC Liberal legacy and expand on BC's biggest boondoggle.
To cancel carries risks, but if the NDP is to deliver on an agenda of 'not-being-the-same-as-the-other-side' then this is the choice they must do. This is what leadership means.

* Full disclosure: NDP has not yet committed to halting the BC Hydro dividend as perfected under the BC Liberals