My father passed away 9 months ago and my mother is in the stage of anger. Any advice?

When my father was still alive, he sometimes was awful to my mother. He verbally abused her at times and it made my mother sick mentally. He often cheated behind my mother’s back and likes to ‘show off’ his mistress to my family. But there are also times that he’s a goofball & a total loving Father.

Before my father passed, he was having an affair with a younger woman and this made a big problem against his side of the family. His side of his family supports his affair (my dad's family doesnt really like my Mom) while my mother’s side disagrees with it (the affair.)

When he passed, my mother was still heartbroken because of the affair. She talks illful about him and sometimes she was ‘grateful’ he was dead. She even rejects to visit my Dad’s grave once in a while (its sort of a tradition in my family to respect the dead/visit the grave once in a while) Then, she started seeing a friend of her that is already married. She states that he is only a supportive friend, but I’ve seen them together and I know they are more than friends. This hurts my feelings and I often cry without my Mom knowing. She hangs out with him since my Dad passed and I dislike the guy because I feel my Mother is trying to make us happy again with his presence. I do not feel happy at all. Am I wrong for feeling this way? How should I act in front of my Mom’s friend? I just dont want my Mom to keep her anger towards my Dad for so long, especially when he’s already dead.

Thank you for all your response in advance.