On the death of her spouse she receives a step up in cost basis of everything that he owned on his death. The thing is he didn't own the trees. The partnership did. He owned a share of the partnership.
So what you need to adjust is her basis in the partnership. That adjusts to the fair market value at his date of death. And if she were to sell that partnership interest then she would be able to use that stepped up cost basis. You adjust this through the partner information section of TurboTax.
Determining the fair market value of your partnership will probably involve using the calculation you have for the land and the trees. It will also involve whatever else the partnership owns. But it won't change her share of the net capital gains.
@KevinP1
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