I hope you do test how it looks on windows? I've worked with a webdesigner that often chose fonts that worked perfectly on his mac, but looked really ugly on all windows browsers, since the fonts were unavailable and the substitutes had noticable style/width differences.

Well, it really depends on how the digitization of Helvetica that any particular user has installed is hinted. The Helvetica that comes with MacOS is quite nicely hinted, and will make for a pleasant read at most sizes.

On Windows, however, Helvetica isn't (at least from my experience) as nicely hinted, or at least doesn't play quite as well with Microsoft ClearType. And that's if it's installed, which is less likely on Windows. In that case, if your font stack specifies a fallback to Arial, which is ClearType's best friend, you'll probably be fine.

Suggest he looks at open sans. It's a fantastic screen font that works very well for large headers (especially at a lighter weight), and body copy.