they are looking for a large apartment to share, but thought maybe the board might have ideas of good (inexpensive) places to eat... and any not-to-miss sites to see?
I highly recommend doing a city tour on arrival. It will give a great overview of the
vast city. Sitting on top of one of the double deckers affords the best view all around. The Gaudi architecture is amazing and can easily be seen on the tour.
Can't help with eateries, we were dining on the cruise ship but have you checked Barcelona Craigslist for housing? It might be a good place to start.