Actually, best case scenario is that the broker failed to contact the exchange. That is clear negligence, and regardless of the merits of the claim, and gives a good chance of recovery. The paper trail on this is trivial, he called them after a trade that any jury (which is important because of the preponderance burden) would recognize as not clear intent. If the broker did not follow up in good faith (fiduciary duty) and document it in its entirety, it's game, set, match. They denied him the opportunity...good help them is they cannot produce a complete record 100% in agreement with those supplied by OP's third party vendors (I.e. Email provider, phone provider, etc.) They may have a supplemental claim against a third party, but that's beyond OP...worst they could do at that point is subpoena him to a deposition to verify their version is events.
I suspect this isn't the case, but makes it very simple from a fiduciary liability standpoint.
