dylan, you may be in for a fun evening.

You will need some corrugated cardboard cut to the same size as your girlfriend's bottom.

From there you will have to ask her to strip down to her skivvies (or less) and sit on that corrugated cardboard... it will take an impression of her Ischial Tuberosities which are also known as sit bones and you you need to measure the distance between those.

This will determine how wide the saddle needs to be, women's saddles also have shorter noses and setting it at the right angle will also make more difference than anything.

I have customers who hated their saddles only because they were improperly set up, my daughters ride on Terry saddles and my wife, who has a very wide sit span can only use a Brooks (and loves it).

When I am doing fittings for women my wife handles the discussion on the girl bits and how to keep them happy and she is very good at it.