Countries have used foreign issued currency before; while it's likely that the same impulse that led a country to leave would also lead it to adopt is own currency, there's no fundamental reason that would have to be the case, they could afford one (or more) existing major ) or even minor, though that's less likely) currencies. That obviously leaves them at the mercy of monetary policy focused on others needs, but it may also reduce uncertainty due to lack of established trust in the new government translating into volatility of the new currency.