What should a wife do about her husband who masturbates and does not perform ghusl afterward, knowing that he prays and fears that his secret will be discovered, and is she sinful if she remains silent?

Masturbation and prayer without purification are both forbidden. If you are certain that he committed these two reprehensible acts, then you must advise him in a way that does not lead to a greater harm. However, if it is merely a suspicion, then it is not permissible to accuse him.