What is the ruling on the action of a friend who exploited his friend's efforts in searching for a job opportunity and its details, then took the job for himself without his friend's knowledge, and is that considered a betrayal of trust and a breach of covenants?

Your initial agreement with your friend before traveling entails that he fulfill his promise and return the favor. His separation from you at work or dissolution of the partnership is permissible, because a partnership contract is permissible and not binding, and each partner may terminate the contract whenever he wishes. Fulfilling a promise is recommended (mustahabb) and not obligatory (wajib) according to the majority of scholars. So, whoever abandons it misses out on virtue and commits a severe disliked act (makrooh shadeed), but does not incur sin. Judging him as having betrayed a trust requires knowing the details of the situation and hearing from both parties.