Just based on the information you have given, I think your mother is referring to a commonly held belief in Southeast Asia that the Koreans were worse than the Japanese during the Japanese Occupation.

During World War 2, Korea was a colony of the Japanese Empire and a still undetermined number of Koreans was forced/recruited to support its war efforts in Southeast Asia. The most famous example is the sex slavery of anywhere between 50,000 and 200,000 “comfort women”, less well known is the recruitment/conscription of Korean males as labourers, soldiers, prison guards and members of the dreaded Japanese military secret police, or Kempeitai (the unit responsible for horrific torture of anyone remotely suspected of being “undesirable elements”).

There is a belief in maritime Southeast Asian countries that were under Japanese occupation that, in their roles as prison guards and Kempeitai, the Koreans (under Japanese supervision) were even more brutal than the Japanese. The belief is mentioned in passing in some of the sources I have seen on the Japanese Occupation of Singapore and Indonesia. A 2012 paper also found that the belief was persistent throughout the Philippines, from Luzon to Mindanao.

The belief goes that Korean and Formosan (Taiwanese) conscripts were looked down upon by the Japanese, thus they were treated especially harshly (Japanese corporal punishment was notoriously brutal), and in turn treated the prisoners under their care in similar fashion. In the case of Formosan conscripts, there is a special sense of betrayal among the Chinese community, as it is alleged that their knowledge of Chinese dialects meant that the Japanese often deployed them as interrogators, and took their advice on which Chinese to kill or torture.

This has not been studied in any great detail (at least as far as I know) so I cannot speak as to its veracity. There were Koreans who were tried by the Allies for war crimes in Southeast Asia (2 were sentenced to death by the Tribunal in Manila), so there were definitely Koreans who committed atrocities, however it's unclear whether they were, in general, actually "worse" than the Japanese, or how much their Korean ethnicity actually affected their actions or their treatment by the Japanese. After all, by this time, Japan had been in control of Korea for over 30 years, and a whole generation of young men had grown up knowing only Japanese colonialism, speaking fluent Japanese and adopting Japanese names.

The belief definitely exists, however, and may be the source of your mother thinking that “Korea” committed atrocities in Indonesia, especially since it is mentioned in comparison with what the Japanese did during World War 2. I am not aware of it being examined in any popular media (like a video on the internet), but I cannot rule out the possibility.

Yu Jose, L. N. (2012) The Koreans in Second World War Philippines: Rumour and History. Journal of Southeast Asian Studies, 43(2), pp. 324-339

Yap, F. (2012). Prisoners of War and Civilian Internees of the Japanese in British Asia: The Similarities and Contrasts of Experience. Journal of Contemporary History, 47(2), pp. 317-346.

José, R. (2001). War and Violence, History and Memory: The Philippine Experience of the Second World War. Asian Journal of Social Science, 29(3), pp. 457-470.

Jenkins, D. (2009). Soeharto and the Japanese Occupation. Indonesia, (88), 1-103.