How do I set a default currency for a specific customer?

Open the customer's profile and set the Currency field. Every new invoice or estimate created for that customer will use that currency by default, regardless of your business's primary currency. You can still override it on the document itself if a specific invoice needs a different currency.

For the full list of per-customer defaults (currency, language, payment terms, late fees), see Setting defaults for individual customers. For how exchange rates work once you start invoicing in multiple currencies, see Multi-currency invoicing and exchange rates.