Your dad needs to consult with an attorney. He should probably start with a family law attorney (most initial consultations are free) and get a reference from them for an attorney to handle the partnership/corporation questions, as needed. The family law attorney may be able to address them.
Who owns the other 2/3's? Your brother? Or are there other partners as well?
I would guess that an audit of some sort (to determine the value of the company) would be required so as to ascertain the value of your dad's share. Then, when allocating marital assets, the value of the company would be included in the division of property. Dad's share of the company doesn't have to be divided and split between him and your mom, but the VALUE of it must be included when awarding ownership of the assets "of the marriage". For example: If his share of the company is valued at $40,000, and the rest of the marital assets are valued at $200,000, then the total value of the marital assets is $240,000. If the court awards a 50/50 split of the assets, then mom would get $120,000 worth and so would dad. His share could include the $40,000 share of the company plus another $80,000 of the marital assets.
Much like a house cannot be cut into two pieces and awarded half to each. They determine a value for the asset, and then proceed to divide the VALUE of the assets. How that division is accomplished can be done by agreement, or if the parties cannot reach an amicable division of property, the court can make the determination.
It would be up to your dad and his attorney to convince the court that the proper division of property would keep that share of the company whole and award it to your dad and not to mom.