While Gnome can be shaped into something Windowslike with extensions 'Arcmenu' and 'Dash to Panel', for another user I'd suggest just using a DE that already works like that. Best choices IMO are Cinnamon or KDE.
Since you already want to use a Ubuntu desktop, I'll stick to suggesting Ubuntu flavours. KDE on Ubuntu, you'll want Kubuntu. For Cinnamon, there is a Ubuntu Cinnamon flavour.
KDE is somewhat more modern-feeling than Cinnamon but also uses more resources. If your GF's system is a bit weak, maybe go for Cinnamon. Cinnamon is also a bit retro in it's design, so it might be more comfortable for people used to older Windows versions.
I recommend downloading a live installer for both, showing her them and letting her pick the one she likes the most. (On that note, I hope you got her permission to do this...)