This was 30 mins into the movie and on first watch I couldn't suspend belief that anyone in their right mind wouldn't NOPE out right then and there.  
  
Up to that point, the evening had already been outlandish enough to send Bear googling if OWW actually works.  
  
Still, a generous viewer could try to rationalize it: Nikki had been pounding tequila shots, she was spiraling from the news of her dad's cancer (or so he thought at that point), and the emotional cocktail might explain the erratic behavior.

But then came the dead cat shrine.

Bear had known Nikki since childhood. In no universe, under any conceivable set of circumstances, is that remotely acceptable behavior. It should have triggered every red flag in existence that she needs serious mental health intervention.

He then talks to Ian, almost giddy, "you won't believe the crazy night I just had dude", clearly excited about Nikki throwing herself at him. Tho to his credit he does mention:

"I think Nikki is going through something."  
"Her dad has cancer, but I think there's more"  
"I think she's having a mental break down."  
"She would freak out and cry and snap back."  
"I think she needs serious help."  
  
But notice how he conveniently left out the cat shrine? I think Bear KNEW if others found out about this, it would immediately pull the plug.

After hearing Ian's theory how she could have been on MDMA, this gave him a lifeline. He googled MDMA side effects: “Impaired judgment.” “Emotional instability.” He clung to that as if it could possibly account for building a shrine to a dead cat but.......

Any 20-something knows that no amount of alcohol mixed with molly turns a childhood friend into someone who thinks a macabre cat altar is a good idea. Bear wasn’t oblivious, he was willfully blind. Choosing denial over doing the right thing.   
  
A supposedly decent guy could look at all those blinking warning signs and still chose to ignore them. Already 30 minutes in.

I rewatched it and Bear is worse than I thought...