Eh, so you mean their requirement is to use this menu?

 https://simplemenu.codeplex.com/

Or is there another Simple menu I don't know of?
That's a menu module, and you should not use that as the main menu component for you DNN site IMHO.

With the DDR menu you could almost generate any HTML you want (if you know XSLT) although IMO that's not needed here.
Getting the menu to also handle 7, 8 or 9 items might be a challenge, also in plain HTML, but that's for later..
(I would solve that using JS I think)

I think without a custom menu template I would float the LI's individually like this;

#nav:after {content: ""; display: table; clear: both;}

#nav li:nth-child(-n+3){float:left;}
#nav li:nth-child(n+4){float:right;}

HTH