I dnt know where your girlfriend is from, but there are certain places which trigger extra checks- this is as people se the "passing through" the UK excuse to gain entrance into the country then dont leave. The authorities look for "risk" triggers, country of origin, type of ticket, how much money you have etc, history of overstaying visas in other places; failing one or more of these they then tend to do a bit of a search of your paperwork and belongings to see if they can find evidence to support or disprove your claim that you are passsing through. I.e letter from someone in another coutry with the dates and details of when you are arriving will support your claim of pasing through, on the otherhand, a letter from someone in the UK, discussing plans for several months time, where you will live, someone who will give you work etc. They then use all of this to decide if they consider you a risk for illegal immigration.
They can do this, its no scam. And it sounds lke they didnt like some of the answers your GF gave or info she had on her.