Navigation Spacing on Browsers different

Site URL: https://chicken-groundhog-daem.squarespace.com/

Hi,

I am wanting my navigation links to break into 2 lines: "Home", "About Us", "What We Do", and "Contact Us" on the first line, and all of the lenses on the second. It works on my computer, but my friend viewed my webpage on his computer and it's not coming up as the 2 breaks. Why are these different?

https://chicken-groundhog-daem.squarespace.com/
password to view: lightsactionco

Add a blank item after Contact Us, then add this to Design > Custom CSS

.header-nav-item:nth-child(6) {
    width: 100%;
}