I'm not sure what exactly he means by "changing the rear derailleur". Shimano road derailleurs do not go to 34 (not officially). Most Shimano mountain derailleurs are incompatible with road shifters. Maybe there is one or two low-end mountain RD models that would work. Maybe the one currently on the bike is medium-cage (it likely is, since the current cassette is 12-30 and short-cage 105's go to 28), and he hopes to make it work with 11-34 by manipulating the configuration screws.

That said, if the guy says he can do it, it probably means he can do it, and 11-34 is nice if you're not skinny and you have too many hills around.