it is a win/win.
You only get half. Half-win for you, n'est-ce pas?
If a glass is half full of beer, is it half full or half empty? A good attitude will tell you it's half full, but there's still the fact that that guy walking away drank the other half, and that might rankle a homebrewer.
Really, I'm just kidding. In fact, I have the exact same situation going on tomorrow. Pretty good friend of mine is coming over, we're brewing 10g of a Belgian quad, and he's taking half. Actually, I will continue to incubate his beer in my equipment, and he will come over and bottle when it is time. He is paying for everything. I will do this once, but that's it. I can't be a surrogate mother... it hurts too much in the end.