Hi,
Well, there is no option to display the price in a different currency in the backend products listing for example. Adding would require custom coding. Custom coding is harder than configuring a few mass actions.
You could pre configure the price changing mass actions for him, and he would just have to change the multiplier value in the action of the mass action before running the mass action, without having to know how to configure mass actions himself. So, if you setup the mass actions for him, it will be as easy for him to update the prices than changing the rate of a currency.
Also, the day your client wants to support prices in several currencies, you're screwed with using the rates as a price changing system.
Also, when your client wants to use the Google Products plugin to generate a XML of his products data (which can be interesting for SEO), the XML will have the prices configured in the products in the backend while he will want the converted prices in the currency used on the frontend. So your solution won't work without adding some extra customization code to the plugin.
Same if your client wants to export a CSV of his products to migrate to another ecommerce solution.
Also, extensions developed by third party developers for HikaShop might not properly convert the currency of the prices. So it might require back and forth with them to add that support in their extension so that the correct prices can be taken into account, if possible at all.
But if you use mass actions instead, then these problems won't happen because the prices will be in the correct currency with the correct value directly in the database.