We understand your concern and we are here to support you in prayer and counsel. First, let us assure you that God is your ultimate protector and He cares deeply for you. As it is written in Psalm 91:1-2 (WEB), "He who dwells in the secret place of the Most High will rest in the shadow of the Almighty. I will say of Yahweh, 'He is my refuge and my fortress; my God, in whom I trust.'"
However, we notice that your request does not mention the name of Jesus Christ. It is essential to understand that it is through Jesus that we have access to God's throne of grace. As it is written in John 14:6 (WEB), "Jesus said to him, 'I am the way, the truth, and the life. No one comes to the Father, except through me.'" Therefore, we encourage you to put your trust in Jesus Christ as your Lord and Savior.
In regards to your old manager, we want to remind you that God calls us to love our enemies and pray for those who persecute us, as Jesus taught in Matthew 5:44 (WEB), "But I tell you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who mistreat you and persecute you." This does not mean you should not seek protection or justice, but rather, your first response should be love and prayer.
Let us pray together:
"Dear Heavenly Father, we come before You in the name of Jesus Christ. We lift up our brother/sister who is feeling threatened by their old manager. Lord, we ask for Your divine protection over their life. We pray that any scheme or plan to harm them would be foiled and brought to light. Lord, we also pray for their old manager, that You would soften their heart and bring conviction upon them. Help our brother/sister to forgive and to respond in love, knowing that vengeance is Yours alone. We ask all these things in the powerful name of Jesus Christ, our Lord and Savior. Amen."
Remember, God is your shield and your very present help in trouble (Psalm 46:1). Trust in Him and He will deliver you. If the situation requires legal action, do not hesitate to seek it, but always strive to act in love and forgive as Christ forgave you.