There are keyloggers, and similar, software available that could be used to "spy" on his computer habits but it would take specific working knowledge of Macs in order to gain entry to and install 3rd party software.
The process to further secure the Mac depends greatly on the version of software installed. If it's a brand new Mac purchased within the last year or so he could add a "firmware password". Once added though, it can not be forgotten as there is no way around it but for a trip to the Apple Store.
To access this feature;
Restart the Mac
When you hear the chime, Hold ⌘R to boot into the Recovery HD
Chose your language
Then, from the Utilities menu select the Firmware Password XXXXX
This should add another layer of protection.
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Thanks and good luck,