I found a Github project licensed under Apache 2.0 that I would like to use in another project. This project contains libraries that seems to be proprietary and that the owner of the project might have bought to use them.
What are the rights that I have on this project ? Can I use the proprietary libraries and the code in my own project ? Can I modify this project and use it ?
I don't understand clearly what are my rights for proprietary libraries in an Apache 2.0 open source code.
You don't link to the project, so I'm assuming that it's clear which file(s) are covered by the Apache licence grant, and which are proprietary.
Apache isn't a copyleft licence, so there are no grounds for suspecting that the author of the Apache-licensed portion intended to distribute these other files under any free licence at all, nor that (s)he sought the rights to do so.
You may use the Apache-licensed content as you would use any other freely-licensed content; you may reuse some or all of it in your own project, provided you honour the terms of the licence. But the non-Apache-licensed content you may make no use of at all; I would advise against even reading it.
Edit: you have now provided a link to the repo in question, which says that everything therein is under Apache2. Because you've come across these DLLs before, you are disinclined to believe that. I don't think you have any choice at this point but to ask the owner of the repo for clarification. Either (s)he will confirm that they have those DLLs on terms that allow redistribution under Apache2, or they'll confess that they don't, and hopefully fix their licensing statement; but I don't think anyone else can resolve this one for you.
