To change a clients account to be billed in Dollars you first need to enable US Dollar as a currency on your account.

To do this go to Setup -> Billing -> Currencies and enable the US Dollar currency.

Once this is done you can go to the client account in SnapBill.

Click on the Modify button in the Client details table and select USD from the Currency dropdown. Now Save changes.

Any invoices you now generate for this client will be in US Dollars.