In most states, there are statutes that allow interested parties to file probate. If she does not file, she is risking having someone else file first, and then she will have additional issues to deal with.
See an elder law attorney, since this subject can be complex and other issues are created that you may need to be informed about.
I agree with attorney Armstrong. In most states, if the person who is named in the will does not file a petition for probate in a reasonable period of time, then another interested person may file a probate petition. If this occurs, the court may not appoint the named individual as the executor or personal representative of the estate due to the delay in filing. It will ultimately be up to the court, though, to determine which person is appointed, even if multiple and conflicting probate petitions are filed.
If your mother does not file quickly and a probate must be done under your state's laws, then I would seek out an attorney who can file a petition to get things moving. You will want to seek out a probate attorney in your state who can help you with this.
I am sorry for your loss. Depending on how your father's assets were titled, probate may not be needed. For example, if all his assets were titled in joint tenancy with your mother, then your mother does not need to probate his estate and can simply show a death certificate or other documents (depending on the laws in WI) to change the title of the assets to her name only.
Otherwise, probate should be opened sooner than later, but (again depending on the law in WI) you usually have years to file. Perhaps your mom simply needs some time to grieve before she begins the process.