Context is key. In your example, the use of "they" is proper and perfectly clear.

However... When my friend Dan is talking about his non-binary child, "They're coming home from college on Friday" or "I told them they should take their sick dog to the vet" is confusing to the listener (I never said anything about it being "completely inappropriate"). In that context, like most folks, I would automatically connote "they," "them," and "their" to represent more than one person, which makes honoring his child's pronoun preference awkward for both Dan and me. Seeing the confusion on my face, the conversation then has to come to an unnecessary pause, for the father to clarify and explain.

In order to avoid this confusion, Dan resorts to referring to his child as "my oldest" or "our firstborn," which removes gender altogether from the reference. In that case, the child's non-binary identity isn't recognized at all. Who wins in that scenario?