What is the expiation for saying to a female friend: "If you do this, you are forbidden to me until the Day of Judgment"?

Prohibiting what is lawful is, according to the preponderant opinion, subject to the ruling of an oath. So, whoever forbids something upon themselves has the choice between abandoning it or expiating their oath, as Allah Almighty says: (O Prophet, why do you forbid what Allah has made lawful for you, seeking to please your wives? And Allah is Forgiving and Merciful) and (Allah has already ordained for you [Muslims] the dissolution of your oaths).

If a woman says to her friend, "If you do this act, you will be forbidden to me until the Day of Judgment," and there is good in speaking and communicating, then she should speak with her and expiate her oath, based on the hadith: (If you take an oath and you see something better than it, then do that which is better and expiate your oath). However, if she does not do what she swore upon, then there is nothing upon her, but she is feared to incur the sin of abandoning a Muslim if there is no valid legal excuse.