Is it correct to prevent our friend from attending mixed youth conferences that contain inappropriate activities, and for which she travels alone, in addition to them having normalization with the Israeli side?

What you mentioned about your friend participating in mixed youth conferences, traveling alone, engaging in activities involving physical contact between genders, and what she told you about drinking alcohol and engaging in illicit relationships, then her saying that forbidding these unlawful acts is from the superficialities of religion, these are grave errors and abominations.

You are correct in what you advised her. If you have exhausted all means of advising her and her guardians, and it has not been effective, then what she does thereafter will not harm you, in accordance with the Almighty’s saying: "O you who have believed, upon you is [the responsibility for] yourselves. Those who have erred will not harm you when you have been guided." (Al-Ma'idah: 105).