Forex Trading - Margin Calls, a Cautionary Tale

The current ask price for EUR/USD is 1.1903. So the investor buys one euro (EUR) at the rate of 1.1903 dollars per euro. Trading one lot (100,000 units) means the investor pays 100,000 x $1.1903 = $119,030 and obtains 100,000 euros. The investor speculates that the euro is undervalued against the dollar, and turns out to be right. Now what?