I only found out about him not declaring assets this year talking with my mom. She was pretty messed up during this time hoping they may still could have salvaged the marriage.
He's an ex-cop, was a police officer at the time he hid this and committed perjury on the forms. I understand judges do not like it when a spouse hides assets and deliberately lies to the court.
Wisconsin law says perjury - a class H felony - needs to be filed 6 years after the perjury. Can we still go after him after this time, either in regular divorce court, or in small claims, and if small claims is possible, under what charges?
Divorce / Separation Lawyer
Chances are you answered your own question. Contact an attorney, determine where the proceedings need to be filed, and discuss some likely outcomes. Don't try to go the free consultation route. This takes work.
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Chances are that your mother would not be successful in reopening this issue. Even though you just found out about it, your mother knew. In divorce, you usually are only allowed to reopen a case within one year of finding out about the fraud. Further, the statute only allows remedy for undisclosed assets exceeding $500. Even though they may total more than that, the value of each individual asset is unlikely to exceed $500 given the items you listed.
She cannot sue him in small claims on this issue - everything needs to be handled through divorce court. Spouses (even ex-spouses) cannot sue each other for actions arising during their marriage in Wisconsin. As far as perjury as a criminal act, you answered your own question. If the statue of limitations has expired, you are out of luck there too.
I feel badly for your mother but she has waited too long. Better to cut her losses and move on with her life.