Here’s a quick bit of CSS to add a line separator between links in the submenu:
#top-menu li li a{border-bottom:3px solid #40593e;}
Change / add properties to this element. For example, if you wanted a white line a the separator and wanted to change the font-size to 14px, use this code:
#top-menu li li a{border-bottom:3px solid #ffffff;font-size:14px;}
Place this code in the Divi >> Theme Options >> General >> Custom CSS or into your child theme’s style.css file