Filter Child back to Parent(s) without using bidirectional?

What is the "easiest" way to enable filtering on the child record so that selecting a child record will filter the parent(s)?

In this scenario, not all parent records have children.  

I have a mockup with Customer and Payment Method, I'd like to be able to click on a child Payment Method and see all the Customers (parents) who used that payment method.

I can filter parent to child of course (selecting Jane shows Cash, Credit, Debit)

But, what about selecting "Credit" so I can see everyone who used that?

I could add a count of Payment Methods to the Customer data grid, but I have to instruct it to show items with no data or it will hide all the parents without children.  If I show items without data it won't filter either, it will just show counts.

*Granted, in the real world you should not have customers who have no payment method, this is just a mockup example.