I have an even better question....how much of the stuff did he just ignore what MSE told him and kept on installing? How much was an actual failure, IE a drive by or zero warning from MSE, and how much was deliberate PEBKAC?
As a PC builder and repairman I have more exp than most when it comes to bugs and AVs (disclosure, I give customers Comodo or Avira, depending on how big PEBKAC they are) and I use MSE on my gaming system and here is the thing...while MSE will TELL you, it won't yank the keyboard out of your hand and slap your wrists. You can say "I choose to ignore this" and click a single button and bypass the block. Now some AVs very much WILL yank the keyboard from you, in fact I recently stopped giving out Avast because it had gotten SO aggressive that even if you told it that it was a false positive and to let it run? it would just straight up ignore you.
But here is the two things you must keep in mind if you choose to run MSE, 1.- It don't do shit as far as webpages, in fact I don't think I have ever seen MSE block single webpage no matter what was on it, so using a browser that runs in low rights mode is a must, and 2.- It was originally Giant AntiSpy and so that is what it works best on, its not really any good at blocking the social engineering based attacks we see a lot today, the "Hey its your BFF (insert name) on (insert chat client) and I found this great page, just click here!" where the person is then led to a page full of zero days type of attack.
That said frankly you shouldn't be giving MSE to your clueless types anyway, that is what a sandboxing AV like Comodo or one that holds their hand like Avira is best at, what MSE is for is for your non clueless who aren't gonna be doing PEBKAC shit and just want a lightweight AV to scan executables and add another layer to their defenses. It was never designed to be the end all be all, you got half a dozen free AVs that do that particular job VERY well, but all of them do HELL of a lot more scanning and thus take up more cycles, and when I'm gaming or editing audio/video? I NEED those cycles, thanks anyway.
My Win 7 system has been running ME since RTM in Oct 09 and its clean as a whistle, then again I run a low rights browser with ABP (a good 85% of bugs IME come from infected ads), don't run strange executables and don't click on email links either. If you are smart enough to show common sense on the web? MSE is fine. if not? Comodo, Avira, Avast, you have choices.