Such a lawsuit is obviously warranted. However, the question is whether any of the officers with knowledge of this evidence will be held accountable. As for the accuser, it is not clear if the messages contradict her statements to the police. They were clearly damaging enough to result in the dismissal of all charges. However, the woman (whose identity is still being withheld) has not been charged with any crime of a false report.
The evidence was found on a computer disk and is believed to have been reviewed by officers, but not shown to either defense counsel or prosecutors. Obviously, any messages between the accuser and the accused are highly material to any investigation or prosecution.
The Metropolitan police has promised an investigation but this student has spent two hellish years in legal limbo. He is more likely to gain more information from his lawsuit than the police on such a controversy.