I thought about that too. However,

I also worried he might take my former boss' side. After all, he still works there. And he is the only one works there now.How dare he?

Actually, I have another witness. She is kind of supplier, we are her customers. I used to talk to her almost once a week. I told her xxx(part time employee) would take my work when I went back to China for vacation, and I will continue work when I am back. So she knew what happened in our company. However, I have the same concern: Is she willing to tell the truth? After all, she is the sales in her company, and she needs to sell their products to this company.

What's your suggestion?

Thank you for your advice.