I am working, with a co-author, on a data modeling toolbox that will someday be sold. Part of the functionality is the extremely common PCA, which we are doing in a more robust way. Thus, we need a PCA function that has the usual algorithm with some modifications. For non-commercial peer-review academic research, I could easily find PCA code that is already written, modify it to suit my needs and use it, citing the author in my research.
My co-author insists that in this commercial venture, it is entirely appropriate to do the same thing. We can take another author's copyrighted code (ex "Copyright (c) 1997 by blah blah blah", not necessarily a GNU license), modify it to suit our needs, and distribute it with the toolbox we are selling. He says that as long as we cite the original author, it's entirely acceptable.
I'm just as strenuously insisting this is not legal, but have been unable to find straightforward documentation. Can someone point me to unambiguous documentation of this?