Seems like a hassle for something that's only £9.
1). From the figures he has given it is likely that even with normal shipping it'll come under the £18 normal threshold
2). Marking the item as a gift only raises the above threshold to £36
3). Fraudulently claiming an item is a gift when is not can get the vendor in a lot of trouble.
Add to this the fact that if he is purchasing it through Amazon US but it is shipping from a marketplace seller in the UK then it will ship from the UK ... to the UK ... so no Customs involved anyway ...

Just watch Customs don't get a hold of it and hammer you with taxes. I ordered a £180 judo suit from america years ago and had to shell out £70 for it to be released from customs.
When buying things abroad from people I always ask them to tick the box claiming it's a gift, this way you can bypass customs and tax can't be added. I've been using this loophole for years ever since my first incident with the judo suit.