I'm inclined to think that you're probably right about your girlfriend. She may well be someone who is not considerate. So that means you need to protect yourself from her exploiting you. You don't need to insert yourself into the middle of her being inconsiderate of someone else. As Rive said above: this dealer rep is a big boy and can look after himself. Just go ahead and provide the ride as you agreed to do.

Observing how this friend treats others may alter your opinion of her, which I think is quite fair.