Indeed, breaking a promise is a grave sin in Islam. Hypocrisy and double face is one of the worst qualities of man and the cause of breaching vows and ignoring promise. The Holy Prophet has said in this connection, There are three qualities and conditions which make a man hypocrite even if he fasts and prays and is a Muslim in his opinions:
1 Shows dishonesty when trusted,
2 Tells a lie while speaking and
3 Breaks a promise.
Breaking a promise is a sign of hypocrisy if done repeatedly, indeed the Prophet said:
Holy Qur'an:
"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.”
Prophet Muhammad (s) promised a man that he would remain sitting in a certain place until the man returned. That man went away and the sun rose. Soon it became very hot but the Prophet never changed his place. One his companions said: "O Prophet of Allah! What difference does it make if you come and sit in the shade?" The Prophet replied: "I promised to meet him here and not to change a place till he comes, in order to fulfill my promise to him. If he does not come, then it will be he who is breaking the promise."
It is better for you to forgive your friend, no matter how hard it may be, and to advise her/his on the severity of her/his sin so that she/he may ask you and Allah for forgiveness
