Please help me.
How could I get a message to the target person that they are being cheated on and ensuring that no one can trace it back to me. The person doing the cheating creeps me out and feels unsafe, but at the same time I think the target person needs to know.
I can only imagine how conflicted you feel, having this knowledge and needing to tell your friend without upsetting the friendship you have. The "bearer of bad news" in these situations rarely escapes unscathed.
What I would do, in your shoes, is create an email account at mail.com and send your friend an anonymous email message. Include as much details you can without revealing your identity.
Be very careful about how you phrase things, it's easy to slip and provide information that only you might know about and give away your identity as the couple fights about the cheating and talk about the circumstances you describe.
I hope this helps!