1). Do you think your friend would really bother to do that? That's a lot of effort for something kind of stupid.
2). If he does, its not bid deal, DUIs are crazy common and a misdemeanor. I'm sure every university admits plenty of people with DUIs.
3) As a practical matter, don't bother disclosing misdemeanors for low paying jobs like that. Two things will hurt you on those applications: criminal records and education over the bachelors level.
4) Is it common for a retail store to ask for a personal statement explaining a conviction? I have never head of that before, especially for a DUI. I mean what can be explained: "I drank some beer and was going home, and..." I don't know why they would ask, but even if they do and try to pass it along to a university, the university (unless they are fools) will not accept it for reasons of authentication. If its not from you it can be easily altered or fabricated, they should know that.
5) Why are you posting this in a forum about law school stuff? Your issue isn't even legal (and if it were this is not the right place for that either), its policy based. Do universities accept documents from third party sources? They can, I guess, if they wanted. But only an idiot would. Do employers ask for a personal statement explaining a conviction? I suppose, if they wanted, but its a weird thing to do.