SOUTH KOREAN President Park Geun-hye has emerged as a solid and constructive U.S. ally during her nearly four years in office. With coaxing from Washington, she mended a rift with Japan’s right-wing prime minister over World War II “comfort women.” She has taken a tough and principled line on North Korea, holding out the prospect of freedom for its people and punishing the regime for its pursuit of nuclear weapons by withdrawing lucrative economic concessions. She agreed to move forward on the deployment of U.S.-supplied missile defenses despite strong opposition from China as well as domestic critics.

From Washington’s point of view, it consequently can be only disturbing to see Ms. Park embroiled in a domestic scandal that seems sure to greatly weaken her authority during her final year in office — if it does not force her to resign. The president already has been obliged to apologize, shake up her cabinet and appoint a new prime minister because of her relationship with a longtime friend who has been accused of corruption.

The first reports about Choi Soon-sil accused her of secretly advising Ms. Park, including editing some of her speeches. That may sound innocuous to Americans accustomed to presidents seeking advice outside the bureaucracy, but Koreans were outraged, in part because of Ms. Park’s long entanglement with the Choi family. Ms. Choi’s father, the founder of a religious sect, befriended Ms. Park decades ago when she was the daughter of the president.

Prosecutors say the younger Choi will be charged with crimes her father was also suspected of: using her connections with Ms. Park for extortion. Ms. Choi allegedly used her influence to obtain $70 million in donations from large South Korean companies for two foundations and then appropriated some of the money for her own use. It’s not clear if Ms. Park knew of the scheme, but prosecutors said one of her senior aides helped Ms. Choi.

The scandal has erupted at a particularly bad time for U.S. interests. With President Obama headed toward lame-duck status, China has been pressing for advantage around the region, courting U.S. allies including the Philippines. North Korea itself is racing to develop intercontinental ballistic missiles and miniaturized warheads that would allow it to reach the United States with a nuclear strike. Addressing that threat, which the Obama administration has neglected, will necessarily be one of the first priorities of the next president.

Ms. Park’s firm stance on North Korea and willingness to partner with the United States could have made her an important ally in any attempt to head off the North Korean threat. Now she may have less authority in that role, especially if she is forced to invite opposition leaders into her cabinet. If she must resign, then South Korea may be in turmoil until its presidential election next year. In short, the big winners in this scandal may be North Korea and China — while the next U.S. president will inherit one more Asian headache.