If they don't have HTML in their email client, you could attach a HTML version. But maybe then, the recipients don't want and/or don't need that extra data.
So rather stay plain text and include a line saying that if the table looks broken, then the human reader should either adjust their e-mail client and change the font, or they could copy or save the mail and display it in another program where they know how to change the font.
Just interested: what e-mail client do they actually use?