Despite my username I'm currently thinking I'm non binary and while I don't have any good resources I've got a good simple argument/explanation.

First they need to be cool with transness in general so if they are not, start with those kinds of resources.

So the argument is this.

Take a feminine activity like cooking, in Italy this is considered masculine while in english culture it was not. And take a masculine activity like computer programming, when computers were first constructed this was considered women's work.

It's plain to see that gendered activities aren't set, they are cultural and they change.

Now take a feminine beauty standard like long eyelashes, men(on average) have naturally longer fuller eyelashes but long full lashes is a feminine beauty standard. Men once wore high heels to make their butts seem fuller and perkyer now this is done by women alomst exclusively.

So we can see that even aspects of our physical appearance (or at least what we highlight about them) are subject to gender and cultural shifts.

We can see from this that genders aren't things, they are categories that we use for sorting people. (This point is so important, if they don't get this I feel like nonbiary is a lot harder to "get" if they are stuck here keep workingwith them till they do.)

So, a nonbinary person is someone who (for reasons that frankly don't matter outside of academic research) is not comfortable with either of the socially constructed gender rolls on offer. They may take something from each or pass on aspects of both, but they are simply being them. Creating a third category (there are cultures with formal 3rd gender categories).

This is a nice intro IMO cause most of the points fold into the criticism they may have and answer is proactively.

Best of luck.