A spaceship is moving horizontally with uniform acceleration. Inside the spaceship, a body m is pushing against the rear wall. This force is action, and is inertial force. As reaction, and as normal force, the rear wall pushes back against the body. Magnitude of the two is ma.
Above explanation should be valid for one inside and outside the spaceship. Inertial force will be a real force for everyone, and will not be fictitious.
