Secondly, your son may have done his charming best to become the beneficiary of all these clothes, clothes I might add that he wants to wear but knows you won't buy him because you don't find the styles "suitable". He knew full well that he was getting clothes, and lots of them, that you would disapprove of. So there may be an element here of "payback" against you for denying him the ability at the age of 14 (when clothes are very important) to be who he wants to be.
I suggest that you call and/or meet this woman and be honest with her about how you feel regarding this situation. Don't go in with a chip on your shoulder or blaming her for going behind your back or undermining your parental role. Returning all these clothes or allowing your son to accept only one moderately priced article of clothing as a gift might be the best way to settle this dilemma.
Of course, that leaves you and your teen still at odds over what kind of clothes he should wear. Clothing style battles with kids are not battles that I suggest parents engage in. Should your taste in clothes translate to forbidding your teenage son from presenting himself in the world as he sees fit? Why, unless his choice of clothing is too expensive for your family's budget?
If economics is the problem, then we have another type of argument, one that may be solved by his earning some money to purchase these more expensive styles of clothing. But, from what you have said, this is an emotional disagreement on "proper" clothing, not an economic disagreement. I'd search your own emotions for the real reasons that you don't want him to dress in a particular way then decide if those reasons are valid enough to dictate to him how he should look. Thanks for writing.