They aren’t saying that he killed her, they’re just saying that he concealed her death?
Apparently so. But he is vile, and if you don’t think death is suitable as a punishment, then perhaps gen pop will do.
It has happened before. Once Tokyo was gonna celebrate all their 100+ residents only to find most were dead and the family engaged in pension fraud.
Near the end of the article it says she was lying in her own urine and feces, so it sounds like he neglected her until she died, and then tried to conceal it.