The text font size and type of the footers of any email sent by a customer to a recipient, is always subsequently decided by the recipients own email client settings. The customer can of course specify theIr own footer text & size settings in their email clients if they so wish, but they must understand that this is not governed by the mail footer settings in their CP.
However, if they create a footer with an html code, they can specify the font, and there are no limits for choosing what font they wish to use. But the rule described above still stands, and that is that if the embedded code stipulates "use the font named "Arial" and the recipient does not have this font on his computer, then this text will be shown using the clients own email client default setting.
The customer must configure his email client to send out in an HTML format for the HTML footer to display correctly.