He isn't hiring. Period. Lol.
Sure, if this is a job - no problem. Agree with Mr E bear that theunreal should ask seller first.
As far as unethical, well, lets say depending on couple things this could be unlawful on top of that, depending on license of seller.
The short answer is "We don't know the situation". If theunreal was a good friend - no problem, if he was looking to hire a developer - there are tons here willing to aid.
The underlying issue is license. Word, photoshop, etc carry non comercial licenses as well as commercial. If it was made with such a license that deems it unlawful, it is enforceable by law. etc. Even if you made edits on top of a non commercial project, it is still unlawful to use it in commercial.
theunreal, I don't think you need to worry, lol, I'm just stressing a point. If you brought it from a developer, I have no doubt he has business license. In fact you have little to worry about, but I would post in help wanted section as a formal job (even for no pay is fine) just phrase it different.
For example: "I'm looking for a developer to do some work on my game"
Its all about the details. And Yes I am blowing this way out of proportion, sorry for that. Just stressing a point.