From the information given, you kind of have a Hybrid question involving Criminal Law and Family Law. If, Your fathers "wife" picked up the check and cashed it, I'm not certain any crime has been committed. He is still married, since Divorce has only been discussed/contemplated, if they have a joint checking account, the wife could certainly deposit her husbands check in that account and use the funds as she deemed fit.
If she's still his wife and they are not filed for divorce or legal separation, then she still has a right to marital property, which would include your dad's income - unless that "bonus" comes from something that he had earned or owned before the marriage.
More likely than not, it was legal for her to cash the check.
Clark County, NV practitioner.
If the check was made out to your father, and he and his wife have a joint checking or savings account then the wife can deposit that check into the joint account and the funds are now available for her to use.