On AA, if you cancel the return after flying the outbound, this usually triggers a credit or refund for the unflown portion (which is often not exactly 50%, but specific to the actual fares used for the outbound and return which could be different) - it does not reprice the outbound you previously flew.

As to whether that ends up being a refund or credit, the usual interpretation is that the return remains refundable to original FOP unless you fail to cancel the return before departure of the return. Simply flying the outbound doesn’t make the return non-refundable. The goal of these kinds of conditions is to prevent no-shows. That said, I don’t have any direct experience trying to refund a fully refundable paid ticket after flying the outbound, so perhaps others can add their experiences.