Answer: All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.

Whoever made a vow to do permissible act isn’t obliged neither to fulfill, unfulfill nor to offer expiation, for Ibn `Abbas (May Allah be Pleased with them) narrated: "While the Prophet (PBUH) was delivering a sermon, he saw a man standing, so he asked about that man. They (the people) said: "It is Abu Israil who has vowed that he will stand and never sit down, and he will never come in the shade, nor speak to anybody, and will fast.'' The Prophet (PBUH) said: "Order him to speak and let him come in the shade, and make him sit down, but let him complete his fast." [Al-Bukhary No. 6704]. 

The previous narration indicates that the Prophet (PBUH) didn’t order the man to offer oath’s expiation, rather he ordered him to unfulfill his oath; in order to prove that fulfilling vows made to do permissible acts isn’t obligatory upon person, especially if they included hardship. However, if the vow was made to perform an act of obedience or devotion, fasting for instance, then he is obliged to fulfill it in this case, thus the Prophet (PBUH) said earlier: ”but let him complete his fast.” As been clarified in Fatwa (245).

In conclusion, making a vow to give your friend a gift to at the coclusion of her marriage is a permissible act and isn’t an act of obedience or devotion that has to be fulfilled. Therefore, your wife (Questioner) isn’t obliged to offer oath’s expiation and there is no harm in being loyal to her friend by sending a gift to where she lives now. And Allah Knows Best.