Your client would be on the borderline between FCPX and iMovie.
iMovie would be simple and sufficient for most of their needs. However, two things might push me to recommend FCPX.
If they might use multiple cameras or multiple audio sources (not just the sound from the Camera's microphone) then FCP becomes the way to go.
Also, the length of the lectures may be a factor. When you edit in iMovie, you are basically creating a list of instructions that will be executed when you render the movie. All rendering is done in one pass as you share the movie. This means that the real "strike zone" for iMovie is consumer videos of 30 minutes or less. You may be able to get videos of an hour or more if they are simple, low def, etc.
With FCPX rendering is done in the background as you go, so there is no constraint on length.
Just to add to resident guru, AppleMan1958's comments...
Keep in mind that when you buy a new Mac you get iMovie and GarageBand so your client will automatically have those options.
If your client is going to be doing a lot of this kind of thing, I would lean towards Final Cut Pro X as it's a bit more robust in terms of features and capabilities.
There's a free trial of Final Cut Pro X available so doing a hands on comparison wouldn't cost anything.
Although, FinalCut Express may still be available, it has a fairly steep learing curve that could be a factor.