Would your roommates refuse to lend you 5 dollars? If not, why is it wrong to take the 5 dollars from his desk, or take money he left in a common area? Under your reasoning it would perfectly fine, common, even.
"I'm sure he won't mind if I just take a few dollars," seems more wrong than "I'm sure he won't mind if I drink one of his fancy beers/soymilk smoothies/etc".
I think you're missing my point. Yes, it seems more wrong to take money than to take a drink from the fridge.
The researcher in this study is saying "aha! taking a drink is exactly the same as taking money, therefore human beings are illogical thinkers in this scenario!" But it is in fact logical - it's based on the anticipated response of the person whose money/drink you're taking (and if you don't know how it belongs to - the general expectations of people in your society/community).
Also, I'm sure that there are societies in which people never take drinks from other people, and consider it the same as taking money. A comprehensive study would have covered more than an American college campus.
No, I don't think that is what he is saying at all. He is saying that standard economics treats the two as being the same, because it reduces things to their dollar value. The experiment identifies an area where standard economic assumptions breakdown, which means that you can't use standard economic theories to determine the result.
"I'm sure he won't mind if I just take a few dollars," seems more wrong than "I'm sure he won't mind if I drink one of his fancy beers/soymilk smoothies/etc".