Praise be to Allah.
Affliction with the evil eye is something real that may happen to a person.
It was narrated from Ibn ‘Abbaas that the Prophet (blessings and peace of Allah be upon him) said: “The evil eye is real, and if anything were to overtake the divine decree, it would be the evil eye, so when you are asked to wash then do so.” Narrated by Muslim (2188).
Allah has prescribed treatment for affliction with the evil eye, and one of the ways of treating it is asking the person who put the evil eye on another to do wudoo’ [then wash the afflicted person with that water].
It was narrated that ‘Aa’ishah (may Allah be pleased with her) said: The one who had put the evil eye on someone would be instructed to do wudoo’, then the one who had been afflicted with it would wash himself using that water. Narrated by Abu Dawood (3880); its isnad was classed as saheeh by al-Albaani in Silsilah al-Ahaadeeth as-Saheehah (6/61).
It was narrated from Abu Umaamah ibn Sahl ibn Hunayf that his father told him: The Messenger of Allah (blessings and peace of Allah be upon him) set out and they travelled with him towards Makkah, then when they were in Shi‘b al-Kharraar in al-Juhfah, Sahl ibn Hunayf did ghusl, and he was a fair-skinned man, with a beautiful body and skin. ‘Aamir ibn Rabee‘ah, a man of Banu ‘Adiy ibn Ka‘b, looked at him when he was doing ghusl and said: I have never seen anything like today, not even the skin of a young woman in seclusion! And Sahl fell to the ground. The people went to the Messenger of Allah (blessings and peace of Allah be upon him), and said to him: O Messenger of Allah, can you help Sahl, for by Allah he cannot raise his head and he cannot come round. He said: “Do you accuse anyone with regard to him?” They said: ‘Aamir ibn Rabee‘ah looked at him. The Messenger of Allah (blessings and peace of Allah be upon him) summoned ‘Aamir and spoke harshly to him, and said: “Why would one of you kill his brother? When you saw something that you admired, why did you not pray for blessing for him?” Then he said to him: “Wash yourself for him.” So he washed his face and hands, and his elbows and knees, and the sides of his feet, and inside his lower garment and the water was collected in a vessel, then that water was poured over him [Sahl]. A man poured it on his head and his back from behind, then Sahl got up and went with the people, and there was nothing wrong with him.
Narrated by Ahmad in al-Musnad (25/355-356); classed as saheeh by al-Albaani in Mishkaat al-Masaabeeh (4562).
So if that happens, if the one who is afflicted with the evil eye pours this water on himself and washes himself with this water, then he has done what is needed, in sha Allah. If any of this water is left over, there is no stipulation that all of it should be used up.
Based on that, there is nothing wrong with someone else using what is left over of this water, if he thinks that he has been affected by the evil eye from the same person, because what matters is using the wudoo’ water of the one who put the evil eye on them, and that may also be achieved in this manner. So there is no need to ask for more water, especially since that may be difficult or would be awkward for both parties.
It is worth noting that the Muslim may ask for the wudoo’ water of the one who put the evil eye on him when there are visible signs of having been affected by the evil eye. If there are no such signs, and it is simply the matter of accusation and waswasah, then we do not think that you should do that, because it will cause embarrassment to the one of whom this request is made, and will lead to a great deal of gossip about him that could undermine his standing. It also opens the door to waswasah and suspicion, because in principle one should turn away from that and not get carried away with it or respond on the basis of waswasah and suspicion.
And Allah knows best.