Your friend is correct for at least some percentage of your readers. I have no idea how many of your readers use GMail, but having a GMail account does not necessarily imply that the messages are read in Google's webmail interface. I have GMail accounts that I read in Apple Mail just like any other email. In fact almost all email accounts can be read either in the provider's webmail interface or using a email client such as Apple Mail, Thuderbird, Entourage, Outlook, etc. The possibilities are legion.
The possibilities are further increased because many services provide different viewing options for the web interface and using client software on your computer. This means that the same GMail user might see your newsletter differently viewing it on the Google webmail interface than they would see viewing the same message using the email client on their computer. Have you ever noticed a link in a rich text email message that says something like, "Click here if you are having trouble viewing this message."? Those links will invariably take you to a web page where the formatted message can be viewed in a web browser. Those links are there for the very reason we are discussing.
IMO for content as long and complex as yours appears to be, viewing the content via the web browser -- that does not mean via a webmail interface but just as a regular web page in HTML or even better PDF -- will come as close as possible to guaranteeing every reader will have the same viewing experience. With email content there is too much that is out of your control. Using PDF instead of HTML will reduce the variations even further. I receive two or three different newsletter notifications via email messages that say something like, "Your copy of the Daily Quibbler is available for download here." Clicking on the link then uses my browser to download the Daily Quibbler in PDF format.
