I think this is pretty tacky and I doubt an FA would do it even if asked. I have never seen this nor heard anyone ask (except for in the movies). You may be able to get them to make you a cocktail and take it back to your friend, but sending one back is just not cool.

If you really want to do something special for your friend, perhaps take note of their seat and mention to the FA (after the service is over) that you'd like to buy your friend a drink and offer them your credit card. I would bet money that they will offer your friend a drink and forget to charge you, especially if you speak with one of the first class FAs.