Is it permissible to proceed with marriage arrangements after someone else—not one of the couple—has performed Istikhara and seen that the marriage should not be completed, knowing that the couple had already performed Istikhara and felt comfortable with the matter?

If a Muslim intends to do something, he should perform Salat al-Istikhara (prayer for guidance) and supplicate for himself. He should not ask someone else to perform Istikhara on his behalf. If, after Istikhara, his heart feels at ease with the matter, then he should put his trust in Allah and proceed with it, provided he does not have a prior inclination. If he does have a prior inclination, he must divest himself of it, then perform Istikhara, and proceed with what his heart feels at ease with. For Allah will facilitate what is good and avert what is evil.