Your brother isn't being "attacked" because of $7, he's being attacked because of integrity issues. The store manager brought the lack of employee discounts to family members up, he went ahead and did it anyways. Plain and simple, if thats what the company dictates as stealing, then it's just as bad as shoplifting.
Now, a second aspect of this might be that he rang up his mother as well, which at every retail job I've ever managed, is a big no-no. Might actually make it look like he intentionally tried to hide that he was letting his mom use the discount. This is all assumption, cause I dunno now the whole register systems at bestbuy work. If thats the case, well, another employee might have reported it. It would take a lot of time to pull register tapes and see what was bought throughout a day on every register unless a tipoff was made, and if thats how it worked out the manager is basically required to perform disciplinary in a non-biased manner.