Zerif Lite mobile menu issue

HI there,

A friend has asked me to have a look at her site. I have fixed a few issues but for some reason, when I resize the browser or use a mobile to view the responsive version , The hamburger version of the menu doesn’t work – It does appear but if you press on it the menu items appear for a second then disappear making it impossible to navigate the site- any ideas would be greatly appreciated.
Thanks in advance

The page I need help with: [log in to see the link]