How is the client currently accepting orders on their store?
If it’s through the creation of a draft order, you’ll be able to see a section on the right hand side of your screen that says Market, as seen in the image below. If you’ve set up the correct markets on the store, and have ensured that each market has been set to show local currency, then changing the market in this section will allow you to send an invoice to a customer in a local market’s currency.
Alternatively however, if the client isn’t looking to use Shopify Payments you can look into installing a third-party invoice app from the Shopify App Store to help with this situation.
This app in particular looks quite promising, as it states that it can “configure multi-language and multi-currency invoices to send to your international customers”. I’m unsure if they require that stores will need to have Shopify Payments activated for this, however. Because their app has been developed by third-party developers outside of the Shopify platform though, you’re best to reach out to the app support team directly to verify this with them. They’re best equipped to support you with any app-specific queries or concerns.
If the above app doesn’t quite suit your client’s needs, feel free to browse other invoice apps here in our App Store.