It’s for a friend!

I'll be buying a dress for a friend from a shop in Tokyo, and want to avoid getting funny looks from the sales clerk.

How do I say "It's for a friend" in Japanese?

You can just say "purezento desu"- it's a present.

Saus,

You could say "tomodachi no tame desu" (it's for a friend), although Sira's suggestion is probably more natural, and will also indicate that you would like it wrapped nicely.
Having said that, according to a cross-dressing friend who has a girlfriend that works in a clothes shop, the shop assistants generally couldnt care less about who buys what - just so long as you don't try on stuff in the shop. It's when male customers make a big fuss about a purchase being "for a friend" that they usually suspect otherwise. ;-)