Dee loves to landscape her yard, but her neighbor Marvin places a low value on his landscaping. When Marvin's grass is neglected and gets long, Dee will mow it for Marvin. This is an example of

Negative externality

Explanation:

A negative externality is a kind of cost that occurs when a person only have an indirect cost of his decision. In other words, a person making a decision doesn't have to suffer the full consequences of the decision.

A negative externality can also occur when the consumption of something good causes a detrimental effect on someone else. In other words, when you do something and it has an effect on someone else even though it's not intentional.

From the question, the nonchalant attitude of Marvin about landscaping costs Dee, hence, that is an example of a negative externality.