I think people are being way too harsh on this guy. Most people make a habit of using the same username everywhere and it's not a problem until there's a sudden national interest in every cat photo they've commented on.
2 years ago it wasn't even clear that HRC would run, let alone that her emails would become such a large and specifically scrutinized aspect of the campaign. Yes, it would've been better if Combetta had the foresight to avoid this sticky situation, but I don't think it's reasonable to claim he's incompetent because of this quite unusual confluence of events.
His two mistakes were using a known username and calling attention to how "VIP" his VIP was. Some people will say the only purpose for the latter is to brag, but I disagree. People on online forums are frequently both hostile and caustic. A likely response to his question would be "Tell them you can't do that". Combetta's emphasis on the VIP's stature was a pre-emptive attempt to get the help he needed without bickering about whether or not he had authority to shut down the request. Since that was obviously not on the table for a sysadmin trying to earn a living, he needed to make it clear that he was dealing with orders originating from well above his pay grade.
People will also say Combetta should've known this was illegal, which I think is silly too. HRC is a laywer. Everyone in politics is a lawyer. There is no intrinsic moral compass around email retention policies, and even if you say "Hmm, that doesn't sound fair", well, we all know that the law isn't perfect -- unfair doesn't make it illegal per se, and none of us would be employed if we had the full moral latitude we'd like to decline employer requests (for example, there's a good argument that modern marketing as an industry has deep moral flaws, but how many companies can afford not to market themselves? There's a lowest-common-denominator factor that drags everyone down. The option becomes indigence or compromise, accepting that everything is not necessarily as we'd like it to be).
If your client is a bunch of lawyers, and you're an IT guy (not a lawyer), and you work for a contracting firm that treats this like a legit request, you're going to assume that what they're asking is legal. Unless something is fundamentally and blatantly immoral (and I don't believe specific sniggles over email retention laws are), I don't think he should be blamed for complying with the directives issued by his clients and allowed by his firm.
This is an embarrassing time for Combetta, to be sure. Let's not unjustifiably heap it on. As far as I'm concerned, he should be fully covered by respondeat superior.
I say all this as someone who is probably going to vote for Trump and thinks that HRC should be prosecuted.