Self defense will ALWAYS involve more than one opponent and/or weapons. Martial arts are mostly useless for that. You need 6 month-ish total of boxing and wrestling.
What you want is a fun martial art, so I recommend just finding a cheap one. TKD is a LOT of fun and can be really cheap, and it's good for your girlfriend since it requires a lot of flexibility, so she'll be naturally good at it.
If you want to prepare for getting jumped by 4 guys with knives, you're looking for "reality based self defense". You can train for it, but I guarantee you it's not what you and your girlfriend are looking for. It's unpleasant training, and frankly kind of scary, mentally. Part of the training involves building the right mind set which means having an 250+ musclebound instructor in your face screaming at you that your girlfriend is fucking ugly and you're a
and he's going to rape her and then randomly stabbing you with an electrified practice knife that you didn't know he had. You spend time talking about when you are personally comfortable with killing people. It's not a fun thing to do with your girl.
Edit: Or maybe the UFC gym is fun for you, my point was self defense should not be a factor in the decision. Take it for fun and money reasons IMO.