What should we do after learning that the partner is working for himself and has exploited our company's resources and transferred all clients to his own company?

This question falls under issues of disputes and conflicts, and its proper recourse is the judiciary, not the mufti (definition). This is because the judiciary is better equipped to hear all parties to a dispute, ascertain the truth of claims, evidence, and defenses, and then issue a judgment based on that. As for the mufti, he only hears from one side, and what he will mention is by way of general guidance and instruction.

It is obligatory for partners not to betray or wrong each other, as these are not characteristics of believers. Allah Almighty said: (And indeed, many of the partners oppress one another, except for those who believe and do righteous deeds—and few are they!) [Saad: 24]. Injustice and betrayal in partnership are causes for the removal of its blessing. In Sunan Abi Dawud, it is narrated on the authority of Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Allah says: 'I am the third of two partners as long as one of them does not betray his companion. If he betrays him, I depart from between them.'" This means that Allah is with them, preserving them, blessing them, and facilitating their sustenance, and upon betrayal, this blessing and care depart.