That 10k was set up perfectly for him, he didn't have to make any tactical decisions or face any other guys who were ready to run sub 27 that day. Sure it was his first 10k, but it's not like he was under trained for it, he does the big miles and strength work. So he ran his first 10k, he was well prepared, and everything went right. It wasn't about experience or race savvy, it was purely a measure of his fitness and physiological capabilities. It was a remarkable performance, but I don't think the fact that it was is debut necessarily means that he would have run any faster had he run it again later in the season.
So to break it down:
1. There are 2 events he can compete in, the 5k and the 10k.
2. He competed in each event last year while healthy, fit, and facing good conditions (pace, weather, etc.)
3. The best time(s) he ran for the each are considered equivalent relative to the rest of the world.
4. He has significantly more race experience in the 5k.
THEREFORE
I think it wise for him to run the 5k
It makes sense that plenty of people will disagree with my assumptions and come to different conclusions. It's quite possible those people are right, (that's the point of having a discussion) but there's no error in my logic.