Is advising the sale of land designated for building a mosque to cover a family's financial needs considered hindering the path of Allah, based on His saying, "And they ask you what they should spend. Say: The surplus [beyond your needs]"?

A waqf (endowment) is not established by mere intention; rather, it requires verbalization. Therefore, your action is not considered hindering God's path, as long as you advised her to spend the money for her own benefit and the benefit of her family.