For what it's worth, you don't have to delete the entire Helvetica font family, just the plain/book version (possibly bold too, but I've not run into that). Same goes for Helvetica Neue. This is a problem in any app that uses WebKit (Safari, Mail, Google Chrome, etc.). The weird thing is that it's not consistent. I run into the problem daily at work where I'm forced to use Google Chrome. The Helvetica fonts on many websites turn to gibberish, but Safari loads them fine. Disabling all Helvetica fonts in our font manager fixes the problem.