Advice needed: my work is uncredited in a commercial game being sold

I modified the battle system of RMXP for a former friend who doesn't know much about programming. I removed the generic attack command and taught them to call common events before and after battles that set up skills based on the actor's/hero's equipment. I sent over a tutorial project explaining this. This work of mine (although miniscule) was vital to the realization of the game.

Since then I cut ties with the person in question for unprofessional behavior. In an early demo, I was credited for programming, the finished product has this credit removed and still uses my work. At the time the project was conceived my help was under the pretense of friendly favor; we signed no contracts.

What is the best course of action in this situation? The game itself is attracting minimal attention, it's more of a flop. I wasn't considering legal action because of that. Still, my work was essentially stolen and the game features at least one scene where a character based off of a real person is shamed.

Any advice is appreciated.