why would you advise a client to make changes like that to a core theme file?
as soon as you release an update, they will all be gone.
and they will then have to remember a) what they changed, b) where they changed it, and c) even THAT they changed it.
This is not compliant with wordpress themes protocol that I am aware of.
All these kind of things should be done through a child theme – but that seems to be not constructed properly to use a child-theme as intended either.