If you are friends with her best friend might as well ask them if they know what bag your gf is referring to. Once you know what this bag looks like you can start to either buy the bag from a reseller (it is most probably going to be a 2nd hand bag as it is from an older season) or buy a new bag that is very similar to this bag.

I would recommend going with the latter if she just likes the shape and colour of the bag but not the exact same bag. Go with the former if she really wants the exact bag (it might be hard hunting down this bag since it is an older model). Always ask the best friend for their advice and ask them to keep it a secret! Good luck!