"My friend wants to buy that piece of clothing."
So the listener knows that the speaker is talking about clothes. 件 can be used for "pieces" of other things like a "piece of luggage" etc. In many cases it would be obvious (in a shop next to a rack of dresses) and you could just say "那件" while pointing and it would be fine, but in other situations you'd need to stipulate 衣服. It's the same in English
件 (jiàn) = item/component/classifier for events, things, clothes etc.
衣 (yī) = clothes
服 (fú) = clothes/dress/to serve/to obey
衣服 (yīfu) = clothes
We already saw fú in:
服 (fú) = to serve/to obey/clothes
务 (wù) = affair/business
服务 (fú wù) = to serve
员 (yuán) = person/employee/member
服务员 (fú wù yuán) = waiter