She can't. Sure, whoever set up the server may be good, even very good. But security is an ongoing game. So even if, and that's a big if, it was a secure OS, running a secure server program and everything was up to date and configured properly... That was yesterday. Without ongoing monitoring of logs and system activity you don't know if you're under attack, have been breached, or not.
Even if you have someone keeping you up to date, how about simple physical security? Was the server room properly secured? Monitored/guarded 24x7? The rule of thumb is physical access equals total system access. How about the DSL or broadband lines into the home, secure or possibly tapped?
Then there's the problem of EMI and shielding. Was her computer at home only used in a properly shielded room. (aka a SCIF?) How about the server itself, shielded? No? If not, someone a good long ways away could have eavesdropped on everything transpiring.
There are far too many holes in her flimsy explanations and assurances. I would bet good money at least one foreign intelligence agency has every single bit of information off that server.
Thank you for your professional input. I'm a ham radio operator and consider myself a 'low end geek' but knew this smelled from an IT security and shielding aspect. You described well what our concerns are. I too would bet good money that Russia, China and who knows who else has that hard drive. That's why Hillary either has to hand that server over tomorrow or the FBI has to go get it. No quarter.