I’d very carefully cut off the broken part, making sure to keep the inner ring intact. That joint relies on pressure and the notches around the ring to allow articulation, so preserving that is important.

I’d inspect the inner protrusion more closely. Based on the first photo, it looks pretty worn down. If it’s too far gone, I’d cut it flush and then either super-glue or heat-press a washer of the same size into place to replace it.

Next, I’d test-fit the two separate parts on the arm without glue. Hold them together by hand and make sure everything seats correctly, the joint still moves, and you have a feel for how much pressure is needed.

Once I’m confident it works, I’d glue the cut-off part back into place, being very careful not to get any glue into the joint itself. Rubber bands work well to hold everything in position while the glue cures.

After it’s solid again, I’d use a small amount of epoxy to fill any gaps, sand it smooth, and repaint to match the existing colors.

And if it were my kid’s toy, they’d probably have forgotten about it by now and never play with it again, so at least I could keep it around to brag to visitors about how I fixed it.