It's possible to temporarily change fonts by pressing "control +" in any browser. He could also change the font sizes within webmail of most email clients. If he goes to Internet Explorer's options page, and goes to Accessibility, he can override font sizes there. He can also make a global font size change that will change it everywhere by just changing the DPI in Windows.
If changing the font size in Internet Explorer doesn't work, he should try clearing the browser cache. Or try another browser like Chrome or Firefox. He should also make sure to use plain text in email instead of email with formatting.