Ko Yao Yai in Thailand is generally a safe destination even for solo female travelers at night. The locals are friendly, and crimes are relatively low. But like any other places, precautions should always be taken, especially while walking alone at night. It is advisable to stay in well-lit areas where there are people. Avoid deserted streets or beaches as not all paths might have lightings. Carry a flashlight if necessary. While generally friendly, not every local might be trustworthy or respectable, so be wary of overly friendly strangers. It's also essential to let someone know your whereabouts.