Here is the thing:

I am huge believer of getting components of the same gruppo and installing them together just to avoid such a thing. And I am glad that your brother is ok. If I were him, I'd sell the Dura Ace crankset, the 6600 FD, the 105 brifters and replace them all with 6500 components (he might actually come ahead in the equation.) Also, I'd double check on that cassette to make sure that it plays nice with the RD.

Mixing and matching is ok for simple components, like hubs or headsets or seat posts or pedals etc, but the drivetrain has to match as close as possible. In the "good" ol' days of friction shifting, things were simpler, but when you get into indexing, mismatching components is a dangerous dice roll...