Thanks guys.
...It's not so much a matter of "did the employee abuse the system?" It's a matter of "the employee abused the system in the past, but we need to prove it." The destinations were major Asian cities and Hawaii makes no sense. We just need to back that up with more than saying "it's obvious." Using kayak and airline websites it's clear as day that Hawaii is absurd, but we just want some more concrete data.
Flightaware looks good.
I think the easy way to shift the burden of proof onto your traveler is to ask: "By what process did you determine that Hawaii was 'better' than available alternatives". Then the traveler is stuck: there is no such process commercially available that would favor a Hawaii stopover, and any that they'd concoct to cover their trail will be obviously wrong. Let the employee try to come up with a better "made sense" answer rather than assume the burden of proving it "makes no sense"
It also avoids the unprovable question: "what were the options available for sale at the time?" (i.e. there was no BOS-NRT nonstop "back then"). You won't quite be able to document what better connecting options were available on the day that your traveler booked the Hawaii connection, but you shouldn't need to because:
1) Options today are roughly the same and you'll be able to see that connections via hubs:
a) come up automatically in online booking sites
b) are shorter by miles and air-time
c) offer faster connections
d) tend to be cheap compared to constructing your own
e) are plentiful (many competitors and many good connecting opportunities)
2) Hawaii connections are none of these, especially if going to an Asian commercial center
a) You have to build them yourself, because booking engines exclude them from top results (or entirely) because they are sooo inferior.
b) Add up the miles or hours and Hawaii will be longer on nearly 100% of trips by about 2 hours of air time, and probably 2 hours more of connecting time (because it forces an unneeded connection probably a 3rd or 4th compared to the many 1stop and 2 stop options.)
c) Hawaii trips are expensive because they'd get constructed as "multi-city" itineraries, and therefore priced as, essentially, 4 one-way tickets (or two round trips (US-HNL-US and HNL-Asia-HNL)
e) and rare...at least in the sense of comparison shopping...any traveler would have to pass up scores of trips that are superior on every dimension (time, connection, cost) in order to get to Hawaii...that's the core of why your employee's selections make no sense.