Can a GFCI receptacle be used as a disconnecting means for an out building (fed by only a single circuit) using the test button? I say no, but I saw this done over the weekend at a friend's place and he swears that it was inspected. I don't believe him.
A GFCI CAN be used as a disconnecting means... that is what is supposed to do. This is not up for debate. Whether or not it is acceptable practice is the question.
A little more detail would be helpful, otherwise we'll be answering your question on a lot of assumptions. There is a circuit, starting with a GFCI at the main building, which then runs to the out building? Depending on the specific situation and the wiring in the outbuilding (the fact that it is only one circuit), I could see it being passed by an AHJ.
Greg: 225.36 from 2011: 225.36 Suitable for Service Equipment. The disconnecting means specified in 225.31 shall be suitable for use as service equipment. Exception: For garages and outbuildings on residential property, a snap switch or a set of 3-way or 4-way snap switches shall be permitted as the disconnecting means.