In general principle, if the receiver is not capable of accepting a detachable magazine, it is not subject to the provisions of the AW ban.
However, exactly what constitutes "not capable" is a matter of technical interpretation by the BATF. [b]I[/b] would agree that a welded or brazed nut preventing removal of the magazine is "non-detachable" but BATF might not agree that it renders the rifle "not capable" of accepting a detachable magazine.
Commercial offerings of non-detachable magazine AR's in the past have included a complete absence of the magwell, or permanently, fully welded magazine, as well as one wierd system with rails which allowed the magazine to be released from the locked position, lowered out of the magwell on those rails/tracks, and pivoted for reloading.
If you (or your friend) were proposing to go into commercial production, I'd urge you to send a sample off to ATF Tech Branch for a type determination. For a one-off personal use gun, I might go so far as to say that complying with the letter of the law wouldn't even require welding or brazing the nut... just requiring the use of a screwdriver/wrench combo should be sufficient, and providing that there's no way to operate the mag catch "normally" by just backing off the nut a bit (lose the spring and shorten the threaded section of the mag catch so the nut can't be backed off and the mag catch operated without the nut dropping off entirely).
So to summarize, if you make the rifle not capable of accepting a detachable magazine, it can have all the "evil features" you like since it's not subject to the AW ban. However, simply making an otherwise detachable magazine non-removable does not necessarily render the RECEIVER not capable of accepting a detachable magazine.