I might be security LARPing but if someone doesn’t need to read the code to be able to do his job he just shouldn’t have access to do it.
Person might have been trustworthy when they were hired and they might change their mind at some point.
Even better - they still might be trustworthy but their computer got compromised and someone might be using them.
Maybe some other employee has problems with them and since they have access malicious employee can indicate trustworthy one in some way. Like posting copy of code somewhere online with nickname used by that one.
I know such things don’t happen often but even for mundane things like phone numbers you just don’t give phone number of your friend to a person you both know without asking that friend in first place.