Email formatting comes in three flavors: plain text, RTF (rich text format), and HTML. The Mail app on the iPad will read all three, however emails created in the Mail app are RTF. There is no way to change this.
You can add HTML to a Mail email by copying and pasting it from another app that can create/edit HTML files. The HTML needs to be in display mode (you see what it's going to look like), not the the raw code (the text with a bunch of extra tags that describe how it should work).
There are a number of apps out there that will let you create and send HTML emails. The only one I'm familiar with is
Blog Docs by Resolvica. It's meant more as a way of creating HTML for blog posts, but it can also create templates for sending HTML emails.
From the sounds of things you aren't really familiar with HTML, so this app may be a bit overcomplicated for your needs and experience. Perhaps someone else can suggest something simpler.
I'm not sure why your friend is asking for HTML email. Almost all email apps, on mobile and computers, are capable of reading all three formats. If we knew why, we might be able to offer a better solution.