Show her how emails get mutilated on different platforms and after forwarding or replying.
Desktops and phones handle email very differently, and peoples local settings affect look, size, images, preview, everything.
No one has complete control how an email appears on someone else's machine. Even the big companies like constant contact or signature formatting folks need to simplify and design for the least complicated display. They say it has to do with the receiver's email settings.
With CSS and HTML its unlikely the receiver sees what you want. It's uncommon that someones email is set up to interpret styles as you send them, for security reasons and others.
You can reassure her that you're designing for the most common settings with images turned on, for example, for most common platforms like Gmail, Outlook etc.
If you want to deliver fancy graphics you have to link to them in your email.
Also show her how your design degrades to decent plain text when someone is blocking features.
After forwarding or replying all bets are off but you can assume its reduced to simple black text, images often appear as broken links.
If its one character length that's causing the problem tell her all email subjects or preview lines must be kept under the character count which causes the bump to next line.
The problem is a real one, and it makes designers look bad, because email formats are so inconsistent and non cross compatible.
Responsiveness, css and html do not work consistently within emails. I believe its intentional, someone didn't want them to be like websites, but like plain letters.