I don’t know that you’ll find many At the Gates afficianados here on CAF. Even those of us here who are metalheads tend to shy away from the death metal stuff. Not that I can speak for everyone in that regard, but I have seen such comments made before (and that’s how I feel about it).
In general, many metal bands seem to thrive on utilizing provocative images that sound worse than they really are. Not that there aren’t some severely anti-Christian metal bands out there (particularly in Scandanavia) – there definitely are. But many bands simply employ dark imagery as part of their schtick.
Though I would not necessarily presume a band was anti-Christian without them explicitly saying so, if listening to the music raises those sorts of alarm bells in your friend’s mind, it might be better for him to stay away from it. There are some bands I stay away from for that reason. Not that those bands are satanic or that it’s inherently sinful to listen to them, but if listening to a band leaves me feeling spiritually unsettled, I stay away.