[downandout]

Unless there is more to the story, he won't be prosecuted for accessing an anonymous FTP server. However, they will scour the computers/drives they took (for months or possibly even years), looking for evidence of this or any other technically illegal misdeed. In the unlikely event they find nothing that they can take issue with (this being a security researcher's computer equipment, they'll find all kinds of hacking tools and possibly evidence of other research that could be construed as hacking attempts), in a year or so, he might get his stuff back. If they find anything, he'll face charges for that.

That's how law enforcement in the US works. A crack in the door, in the form of a ridiculous accusation, is all it takes for one's life to be destroyed.