I was eating in a restaurant with my friend, who ordered an "all-you-can-eat" menu item (so he could take as much food as he wants) and gave me some of the food to taste. I ate about a plateful. Normally, the "all-you-can-eat" option is only for the person who actually pays. Did I steal, and do I have to go back and pay the value of what I ate?
The only analogue I can think of offhand (not that I know vast areas of Torah) — that is, where someone has contracted for an unlimited amount of a product within a finite amount of time — is the right of a foodworker to eat from the food he's working with: see Shulchan Aruch, Choshen Mishpat 337. Paragraph 18 there says he may not give of the food to others, and the S'ma (38) clarifies that it's considered theft and that he's required to pay for the amount stolen. (He does not say the recipient is required to, which would be the analogue to your question.)
Of course, the analogy is imperfect, but it's pretty close, I think. As always, consult your rabbi with any practical questions.
 For one thing, an all-you-can-eat bar is usually literally all you can eat (or as much as is in stock), whereas a worker is not allowed to engage in achila gasa, excessive consumption. Also, you're paying for the right to eat, whereas the worker is guaranteed that right by the Torah. But I don't see why either of these distinctions should make a difference.
For some reason, I recalled this question a few days ago. (Perhaps, because I was hungry! I rarely eat out, and at home, my wife and I constantly share our plates and palates!)
Here's the answer I got from dinonline.org:
If when your friend took the food he was intending for himself alone to eat, it is permitted for you to taste from his plate.
It would not be permitted for him to take initially with intention of giving to others.
This assumes that there are no clear in-house relating to this matter.