The second photographer is using an 18-55mm kit lens. At 50mm all of Canon's 18-55mm kit lenses have a maximum aperture of f/5.6.
If one is constrained by using ISO 400 and 50mm in order to match the other camera, then the only variable left is exposure duration, otherwise known colloquially as "shutter speed".
Since f/5.6 is two stops slower than ...
Equivalent exposures are:
ISO 400 f/2.8 1/250 second
ISO 400 f/4 1/125
ISO 400 f/5.6 1/60
ISO 400 f/8 1/30
ISO 200 f/4 1/60
ISO 200 f/5.6 1/30
Stopping down one stop of aperture is compensated by one stop slower shutter speed,
OR by one stop smaller ISO.
You probably use a small aperture. When autofocus does its work, it usually does it with wide open aperture. That's why you get a brighter image for a small time. It says nothing about the brightness of the final image. This may be different when using phased-based autofocus and contrast-based autofocus, but this depends on what the camera does.
Why would you commit any more film until you have had the first roll from any film camera developed? Get the first roll developed and see what it looks like.
If it's massively over or under exposed, then dedicate the next roll to test shots where you systematically start at a longer exposure and then decrease the exposure time about one-half stop each frame ...