I don't agree that Hen nor anyone else took Chimney's stabbing lightly. Besides Buck and Athena they were all camped out at the hospital waiting for news of his recovery. That was their focus.
Because Chimney was as stable as possible at the hospital, it made sense that Maddie's brother would be focused on trying to find out if she was still alive so he could try to help her. It also made sense that a friend/colleague who knew how impulsively wrong he could be in that situation (Athena, in this case) would volunteer to assist him.
Again, Chimney was safe and, hopefully, recovering. But Maddie was a question mark. It would not have been enough for Buck to stand by and hope the police got it right with his sister.
Further, it was established early on at Maddie's arrival that Buck had no idea that Doug was abusive. Maddie purposefully portrayed marital perfection to everyone, including her family. It was part of the "abuse theater" she had with Doug.