Randy
It is a good question, one that I think will be hard to answer given the lack of information we have.
It makes sense that the first to die gets buried by the survivor in some fashion, even if that means just getting a bunch of rocks piled on top of them. It would be natural for a survivor to do that, if they we physically able to do so. In this case we simply don't know the timing or status of the first of the two to go, so it is possible that other scenarios played out that leave no trace. Was Fred in the Electra? Maybe. Did he fall and get washed out to sea on his way to/from the aircraft? Attacked by sharks (unlikely)? Is he buried somewhere on the NW end of the island that we simply haven't found yet? It's possible. Was he out hunting food & water and just never came back to camp, now scattered about in some inaccessible place we haven't looked yet? Could be.
Having been there a number of times, I can tell you that it is really easy to get disoriented and turned around in your thoughts about how to navigate through the underbrush, particularly scaevola. It isn't hard to imagine Fred out foraging / exploring and simply getting lost and stuck, weakened and exhausted, out in the scaevola with no food, and no water. AE may have never known what happened to him, just disappeared one day and didn't come back to camp.
People sometimes say things like "haven't you searched the whole island yet?" or "if there was anything there to find, you should have found it by now" but the reality is that there is a lot of territory that we haven't been able to search due to time and manpower constraints. Nikumaroro is much bigger than most imagine, and most of it is heavily vegetated. For those who haven't seen the aerial tour of Nikumaroro, you should check it out.
https://www.youtube.com/watch?v=DL9FGsvB3E8 The combined man-hours of 12 expeditions is still not enough to have searched everywhere. Just last year we found several ancient graves up in the NW end we never knew about. There is still a lot to learn about Nikumaroro.
So, I think that poor old Fred is still out there, probably under a pile of rocks up on the NW end, with a Brandis sextant on his chest, but that is pretty much just my speculation.
Cheers
Andrew