Is it sufficient to perform ablution and give the water to someone suspected of having been afflicted by the evil eye, without informing them?

Your question about sufficiency can be interpreted in two ways:

First: Sufficiency in terms of absolving yourself of any right she might have against you if you caused her harm due to envy. In this case, ghusl (definition) (ritual bath) is not sufficient. Rather, repentance is necessary, with its three conditions, and seeking her forgiveness, unless you fear a greater harm, in which case you should frequently supplicate for her.

Second: Sufficiency in terms of the reported ghusl for the envied person. We hope it is sufficient. This is done by bringing the envier and asking him to perform wudu (ablution). Then, the water that splattered from his limbs is collected and poured over the afflicted person, who also drinks from it. There is another method: to take something from his clothing, or dust if he walked on it while it was wet, and pour water over it, which is then sprinkled on the afflicted person or drunk by them.