What constitutes 'Khalwah' (being alone with the water) that forbids a man from using a woman's leftover water?

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The scholars have differed on defining the 'Khalwah' in this context. Al-Sharif Abu Ja'far held that Khalwah means the absence of anyone whose presence prevents the state of seclusion necessary in marriage (Nikah), whether male, female, or a discerning child, because their presence invalidates seclusion similar to the other type of seclusion. The view of Al-Qadi is that Khalwah is defined by the absence of a Muslim man; the presence of a child, a woman, or a non-Muslim man does not remove the state of Khalwah. Another group of scholars defined Khalwah as the woman using the water without the man participating in its use, based on Ahmad’s statement that if she is alone with it, he should not use it, but if they both use it simultaneously, there is no issue, citing the practice of Aisha and the Prophet sharing a vessel.