Going back to the original post - it seems that the OP does understand the basics of fitting as he mentions the correct relationship between length and lie. Reading between the lines a bit, it also seems that the fitters at DSG understand that relationship as well. The assumption is that they potentially did a lie board test and at least a static measurement to come to the conclusion that a flatter lie was needed. The OP did not mention what methods were used to conclude 3 degrees flat.
Not only do we not have complete info. as to why DSG recommended only 3 degrees flat rather than some combination of length and lie adjustment, but other factors may be involved. DSG may have been thinking about cost to the customer. To shorten ,the grips either need to be saved (5.99$ ea) or new ones bought and installed (about 10$ per club). Bending alone is 4.99$ per or 32$ for eight. Do the math and the combo adjustment gets expensive. Bending alone is the cost effective option and maybe part of the DSG thinking took this into account.
So just for discussion, let's assume that the dynamic fitting (and static fitting) revealed 3 degrees flat and standard length. Then the fitter is doing their job. Let's say it showed 1/2 inch short implying that 2 degrees flat would now work. OK - fine. But other factors like age of the clubs, skill of the golfer, frequency of play, etc. may have made the combo adjustment unnecessary / not preferred by the customer (due to cost). If 3 degrees flat alone works, then DSG can certainly be trusted to make such a recommendation and be reluctant to charge the customer for further adjustments.
In conclusion, we do not have enough info to make a clear determination as to whether the DSG fitters can be "trusted". The OP asked a loaded question and got the usual loaded answers regarding what is commonly perceived to be a big box experience. It would be fair and instructive to get the fitting sheet (hopefully it exists) and describe in detail the conversation between DSG and your friend. If the fitting sheet exists, take a pic and post it, and then all the experts on this site can make a recommendation as to whether the DSG fitter did their job and knows their stuff (can be trusted).
Sometimes customers just want what they want, even when it is not in their best interest. Either way, it would be great to have more info about the entire situation to make a more accurate determination.