I have discovered that one of my clients is a fraudster who is outsourcing work he's paid for by his company to others on Upwork and leaving a factually false review after I found that out. I have proof for this but I am of course not going to be posting private information here. I'd greatly appreciate if someone from Upwork can reach out to me regarding this.
Thank you for reaching out to us. Could you please click on my name and send me a PM with more information about the client you are referring to? I will be sure to look into your report and escalate it accordingly.
This is not quite clear.
Hiring another freelancer to outsource work is not forbidden on upwork, provided the client is aware and it's fixed price.
I don't doubt that there are lots of people doing this without informing the client, but fraud is maybe not the right word, unless there are factors involved you haven't shared. It sounds as if you know who the original client is and were able to contact him/her. It is not really your place to do that, you don't have a contract with them.
So unless there is some fundamental information missing in your account, both client and freelancer did things that were sketchy.