yes like ethel says, shop around as womens shoes can be narrow, mind you someone bought me some size 6 shoes that I could not get my foot into last week but they fitted my size 3 daughter perfectly, they were tiny
If you base it on standard British sizes, then I'd imagine that all size 9's would be the same, but as others've said, they can vary. Best to buy the size which your friend thinks'll be right, and then ask the shop if they can be exchanged if they're wrong.