ANSWER:
If she did not see a wet dream but sees a significant amount of discharge in her undergarment, she must perform ghusl as this was an ejaculation of semen which requires ghusl. If the amount is small, then most likely this is mathi which is impure and only requires washing the affected areas and renewal of wudu or it is her normal vaginal discharges which requires no action at all.
https://youtu.be/tHSx2whow4I