I’m unclear on what you mean—did someone who reviewed the paper inform you of this? If so, that would be private evidence that he was on the paper, though given that you’re an anonymous source claiming private information, I’d want to have someone who is not anonymous clarify what occurred and whose names were on the paper when it was reviewed, even if the identified individual didn’t disclose who the reviewer was.
I’m unclear on what you mean—did someone who reviewed the paper inform you of this? If so, that would be private evidence that he was on the paper, though given that you’re an anonymous source claiming private information, I’d want to have someone who is not anonymous clarify what occurred and whose names were on the paper when it was reviewed, even if the identified individual didn’t disclose who the reviewer was.