Opening an iPhone does not void the warranty. The FTC's Magnuson-Moss Warranty Act of 1975 PREVENTS Apple from voiding the warranty simply due to a customer's opening of an iPhone.
So it appears that Chris is both ignorant of the law and mis-informed. BUT, keep reading!
The link I provided mentions the act, and feel free to find and read the act itself. With credit to OWC, I note:
The US Federal Trade Commission has very specific consumer protection laws, as outlined by the FTC's Magnuson-Moss Warranty Act of 1975.
This act outlines several key points, including the basic consumer protection that a manufacturers cannot void the warranty on components that are not directly affected by the installation/placement of after-market upgrade components.
Reasonably speaking, if you properly install your battery upgrade, nothing being performed should void your warranty. It is wise, however, to know your skill and comfort level in performing a mechanical procedure.
ON THE OTHER HAND: It IS a very tricky procedure; there's a tiny little bracket that comes loose when one unscrews the battery connector, and it's vital to deal with it properly. Big jugs probably didn't deal with it properly. Signal problems result when one doesn't deal with it properly. Big jugs' signal problems probably are directly affected by the installation/placement of after-market battery upgrade, and his warranty should not cover the problems caused.