what does islam say about breaking promises?

السلام عليكم ورحمة الله وبركاته.

Indeed, breaking a promise is a grave sin in Islam, especially if it is to Allah.

"O you who believe! Why do you say that which you do not do? It is most hateful to Allah that you should say that which you do not do.” - Qur'an [61:2-3]

The Prophet's fourth grandson, Imam Ja'far al-Sadiq, gave a tafsir (interpretation) of this verse and said:

“A believer’s promise to his believing brother is a vow that has no expiation (It cannot be broken). But one who goes back on his word declares his opposition and enmity to Allah, and invokes the anger of Allah.”

And Ali ibn Abi Talib, the Prophet's cousin, said:

"Breaking a promise causes the anger of Allah."

The third grandson of the Prophet, Muhammad al-Baqir, said:

"There are four types of sinners who are punished very soon:

1)      The person who violates the pact that he has made with you, even though you have respected it.
2)      One who inflicts harm upon you even though you caused him no harm.
3)      One who has promised you something and you are faithful upon your agreement but he is unfaithful.
4)      One who wants to severe relations even though the relative wishes to continue the relationship."

Breaking a promise is a sign of hypocrisy if done repeatedly, indeed the Prophet said:

“One who has belief in Allah and in last day must keep his word when he promises.”

AND

“Three traits make man a hypocrite even if he prays and fasts:
1) Betrayal of trust
2) Lying
3) Non-fulfilment of promise.”

It is better for you to forgive your friend, no matter how hard it may be, and to advise her on the severity of her sin so that she may ask you - and Allah - for forgiveness,

I hope this was of help,
و عليكم السلام و رحمة الله و بركاته
Source(s):
The Qur'an

Hadiths from:
Ja'far al-Sadiq (alayhi salam)
Muhammad al-Baqir (alayhi salam)
Muhammad al-Mustapha (Sallallahu Alayhi wa Alihi wa Sallam)