It depends on what they do with it.
One of my favorite bands (or at least they were until recently ) did that for a remix album. Of course, that was a remix album so it was pretty much just the Christian band's songs.... remixed. Lol
But I don't really see a problem with a Christian band collaborating with a secular band. Uh, unless they start singing about things that kind of go against everything they previously stood for. I guess that would be a problem.