Your girlfriend needs to bring th ecar to a reputable local shop, show them the noise, tell them about the other symptoms including the CEL, tell them honestly about the long term lack of maintenance, and let them go over the car thoroughly and give her a good overall assessment with cost breakdowns.
The noise, lack of power, and CEL could be a simple no-longer-firing cylinder, or it could be a more serious problem brought on by maintenance neglect, but unless the car gets a good going-over it may be not long for this world. You cannot neglect the things forever. They will begin to fail.
And have the entire car gone over, not just the engine. Things like brake systems and chassis should not be neglected either. If you can't start a car it will ruin your day, but if you can't stop a car it can ruin the rest of your life.