Hey Big, dirk here. I work with the Mammoth Track Club and Meb Keflezighi specifically (you tube us training, I am on the bike in orange : ) Anyway I have great insight and knowledge of how to fit and even TIE your shoes.
#1. Start by replacing the stock “footbed” with a black or possibly green SUPERFOOT. Accept no substitute.
These help maintain a neutral foot position within the shoe by supporting the foot in its anatomically
neutral position. Similar to your hand shape after you shake your hand out.
#2. A proper shoe will fit similar to a proper hand shake or well tailored custom GLOVE. You may have to try
MANY pairs to find the correct shoe. Be prepared to spend over an hour in this endeavor.
#3. Factors such as distance she is training for, type of terrain she is running on AND if she is a heel or forefoot striker along with her weight will all be taken (or should be) into account by the store you are supporting.
#4. this will sound stupid but HOW you tie your shoes can have a significan effect on how shoes not only fit but also perform. To properly tie your shoe, insert foot into the shoe and TIP the TOE up so it is at or above ankle level. This will allow the foot to slip ALL the way back into the heel cup. At this point start to tighten your laces from the toe area back towards your heeel. With this you will get a proper fit.
#5. Last point, 10K and longer distance runners tend to go with a shoe 1/2 size larger to account for foot swelling. But remember that this step MUST meet the first FOUR requirements.
I hope that this helps you and your girlfriend in being able to run pain and injury free. Sincerly killerDIRK.