What you really want to do is some compositing work in After Effects or in FCS- Motion. You're masking out one half of the frame, and compositing the other half of the next frame within it.
You should do the Motion tutorials. This is a relatively easy thing to accomplish, but you'll learn how to do it properly in the tutorials.
In the short term, you can do a very quick and dirty version, like your friend did, by Cropping or Masking out part of the frame in the FCP timeline. This is one way to do that- probably the dirtiest. Again, to make this effect work properly, you really need to do it in other compositing programs. Until then:
1. Put the clips in two video tracks (V1, V2). Put one clip in V1, and the second clip directly above it on V2. make sure they are synced up how you want them to be.
2. Double click the clip in V2 to bring it into the Viewer window.
3. In the upper area of the Viewer window, click the "Motion" tab.
4. Look for "Cropping"
5. Tweak the left and right, or top and bottom cropping sliders until you've cut out the correct section of your frame. Note: If you have your playhead on the same frames in the timeline itself, you'll be able to see your adjustments in real time in the Browser window. So you should put the playhead in the clips you're working on.
That's it. You may have to render the video in order to see your work.
Tips:
1. Use the "edge feather" selection to tweak the "softness" of the crease on the cropping point. This will help to make it more invisible.
2. It's probably obvious, but for this effect to work the best, you have to have an identical frame for both clips. So if your friend jumps behind a tree and reemerges, the camera has NEVER MOVED. that will make the seam as invisible as possible.