If you want human and not just humanoid, then the advice about convergent evolution in that other question isn't really applicable - getting the basic shape is certainly plausible, but a near-perfect match starts stacking up too many coincedences. You essentially have three options:
1) It's not a coincedence
There's some kind of link between the Earth human and the alien 'human'. Maybe the 'alien' is the descendant of previous kidnap subjects who were transplanted to this other planet, maybe both Earth and the other planet were seeded with life as part of some alien terraforming process, maybe the aliens were deliberately shaped to resemble humans for some reason (or earth humans were shaped to resemble aliens!), maybe there's some cosmic... something... that pushes intelligent life towards evolving into 'human' form.
Whatever it is, the alien somehow looks like a human for a reason, despite the distance between Earth and the alien planet.
2) The universe is a REALLY big place
Getting (almost) exactly the same result in two places independently isn't impossible, just very unlikely. If you roll the dice enough times, even the most unlikely coincedences have a chance of coming up eventually. Note, however, that for this to make sense on its own you need incredibly large numbers of alien races - enough that it would be impossible for a human to meet more than a tiny fraction of them in one lifetime. This shifts the question from 'why does this species exist' to 'why did he happen meet one', but fortunately that's a little easier to justify. Introducing the alien to someone who looks like him is an understandable impulse.
3) Probability? What's that?
Yes, it's freakishly unlikely. It happened anyway, and you don't feel like discussing the matter further. (This is not a recommended option, but as noted above 'impossible' and 'unlikely' - even 'very very very unlikely' - aren't at all the same thing.)