Pretty embarrassed to even admit this, but there was absolutely NOTHING wrong with the car. Aside from it being out of gas. Yup, the f*cking car ran out of gas. My fiancee says that she had roughly 40 miles left to empty when this happened, which is the primary reason I didn't even consider the fuel being an issue. I've definitely learned my lesson and not to rule out or assume anything.
We had it towed to BMW of Newport yesterday morning. After offering to give Katie a loaner, they call her back shortly after and claim that the car is running like a champ. And that all they had to do was add some gas. The guy claimed that the "miles to empty" is only accurate to about 50 miles. Not entirely sure how much I believe this, however, since I've never had an issue with this on my car. Nor has my buddy with an e46 330i. But from now on, I've instructed her to fill up when it hits a 1/4 tank, just to be safe.
So yes, it was a very embarrassing situation, but the service dude that helped Katie out was super friendly and they didn't charge the $65 diagnosis fee, which was very surprising. They even washed her car. So needless to say, they took care of us. Katie will be mailing them a hand-written thank you note in the very near future.
Crisis (ahem, new fuel tank) averted.