I don't think you need to spend $400 bucks on a sleeping bag for this trip.
True enough. My main point was to contrast that a high-end bag would have an accurate temp rating, but if he buys a mid-range bag he should take the rating with a grain of salt and shoot for a lower degrees rating to make up for that. Also, that, given a choice of dropping a lot of money on a GPS/electronic gadget or a sleeping bag, the sleeping bag would be the right investment.
Some less-expensive, but still good, bags to consider would include (in somewhat descending order) Montbell, Marmot, REI, or Kelty at the lower end. Basically, if the bag will keep you warm and has reasonable workmanship, the main variable will be that the less you pay, the heavier and bulkier it will be. As Oregon Mouse pointed out, girth is an important consideration, too.
I generally carry an REI Sub-Kilo rated to 20 degrees that would more accurately be called 25 degrees. It has a narrow girth, but I have narrow shoulders, so it works well enough for me. It cost me in the neighborhood of $130, on sale at the REI-Outlet. It still shows up there from time to time.