Do you know what he is using to waterproof the fence?
makes an interesting read, some of it quite alarming but
"Research on the use of CCA-treated lumber in gardens has shown that treatment chemicals do not affect the growth or safety of home-grown produce."
In actual fact the sealant could well be helping with any safety issues with the treated wood. Since it would probably keep the chemicals in the treated wood locked away. So it is actually a good thing that he treats his fence regularly.
On the other hand, how do you enjoy the veg you grow if in the back of your mind you are worrying about toxic chemicals? Even if there is no risk you still will not be happy. If you use weedkillers normally in the small print is a recommended time a few weeks where it says not to eat the veg in that area. So even where there can be a hazard after spraying it doesn't mean it will be for long.
I would be tempted to grow flowers and shrubs in that section of the garden or grow in containers that way you can feel confident about the food you produce. Although in actual fact there may be no risk at all.
To put your mind at rest, you really need to know the product that he is using to be able to judge if there is a possible hazard and the steps to avoid any harm.