Under normal circumstances the term OAT means SAT but who knows? Also air temperature gauges on aircraft may show SAT or TAT, there is no rule and you cannot really deduce whether the temperature is SAT or TAT by looking at the ISA and saying 'that's too hot'.
With the data given my interpretation is the same as that of the OP, OAT is given and the temperature rise is irrelevant. Some of the online calculators give results too far apart to be serious, the old CRP-5 gives me a first TAS reading of 469KT without compressibility which corrects down to 455KT, as the OP says.
Suggest OP asks his colleague why this is wrong!