I had the same problem with my g/f at first.....

So what I did was to start riding beside her on the road while she was on the sidewalk... pritty soon she figured out that the sidwalk was not really meant for bikes... lots of close calls with obstacles, poles.. one of the worse issues was sidewalk exit and entry ramps... she soon realized that I was not slowing down like she was at each intersection.. I would ride through, slow down, and wait for her to catch up almost every single time...

Now we ride exclusively on the roads.. it took her about a month to transition fully but she finally did...

One thing that helped was that in some parts of our neighborhood we don't have sidewalks at all... so she just had no choice. Part of our weekend ride is 60% road (without useable sidewalks).


Good luck !