Are they sinful for taking and eating the fruit that the husband took from a truck driver who claimed he found it in one of the fields?

If the driver picked the fruit from a public, unowned field, then there is no harm in your husband benefiting from it, based on the Almighty's saying: "It is He who created for you all of that which is on the earth."

However, if he picked it from an owned field without permission, then this is theft, based on the Prophet's (peace and blessings be upon him) saying: "The property of a Muslim person is not lawful except with his good will."

Therefore, your husband must return the fruit to its owner if he knows him. Otherwise, he should give it as charity on his behalf, for there is no benefit in withholding wealth.