Hi,
If someone uses a coupon configured with a flat amount in a certain currency while having his current currency being different, the flat amount value of the coupon will be converted to the current currency of the customer using the currency rate / fee available when you edit your currencies in the menu System>Currencies.
The total amount of the order will still be in the current currency.
Also, if you don't use a "flat amount" in your coupon but a percentage, then it won't matter as the value of the coupon will be calculated as a percentage of the total amount in the same currency.