The trouble is Cohen has no evidence to support his claim... unless he can describe a meeting in detail where the info was discussed and Mueller has a second witness to cooberate that meeting's details.
In terms of a federal courtroom, his testimony
is evidence. First, it's clearly relevant to whether Trump knew about the meeting beforehand. Second it's more probative than prejudicial in that rather than misleading the trier of fact or confusing any issues, it serves to clarify the most important issue as to Trump himself. Third, Cohen has personal, firsthand knowledge of the occurrence.
That gets you through the first several steps of admissible testimony. The next issue is hearsay.
Assuming that Cohen was there when Trump agreed to the meeting, or that Trump personally communicated this information to Cohen, there are multiple exceptions and non-hearsay statements that could overcome the rule against hearsay: statement by a party (or statement by a party opponent), a statement against interests (penal or pecuniary), a statement by a co-conspirator, prior inconsistent statement, etc. If Cohen then took some action based on Trump's statement then there's what's called "effect on the listener." If Trump made mention of speaking with e.g. Trump Jr. after the meeting then it shows present state of mind where intent or plan is involved.
There's no requirement that witness testimony needs independent corroboration here. There are narrow circumstances under which some testimony under certain circumstances must have corroborative support, but this is not one of them.
So then we go to impeachment. Cohen could be impeached like a motherfucker for dishonesty. That is, any attorney would be able to cast doubt on Cohen's credibility as a witness, primarily by showing bias (e.g. you're doing this to save your own ass, to exact revenge, to Whatever). And one way or another, all the stuff he's in trouble for now would also come in even though the general rule is that prior specific acts are not admissible to show character in conformity with conduct in federal case unless character is directly in issue. To be clear, when a witness takes the stand, their credibility is always in issue, but not their character--there's a difference. Credibility goes to truthfulness, while character goes to showing that the witness acted in conformity on the occasion in question as they have in the past under similar circumstances.
What it would come down to is the proponent attorney rehabilitating Cohen after cross examination and seeing to it that even though Cohen's a sleaze-ball, his testimony on this issue is credible.
So the bottom line here is not corroboration. It's certainly helpful, but it's not necessary.
After all that, this isn't about what will happen in a courtroom. What it's about is whether Mueller can effectively use this as part of a case to proffer that criminal activity was afoot with respect to a conspiracy to commit illegal acts. Thus, he would indeed have to take into consideration all of the above (and more) before suggesting that this would be a case worth trying. It's quite reasonable and probable, assuming Cohen was willing to testify, that Cohen would be an important witness in the prosecutor's case in chief.
