HUGE Cunard enthusiast here. Done two trans-Atlantic crossings, one on QE2 in April 2008 and one on QM2 this May. Have another crossing booked on Queen Victoria in January, thinking about one on QM2 in June 2010, and am doing the maiden crossing on the new Queen Elizabeth in January 2011.
First off, dining is all included, as is room service. The only extra food-related costs are drinks and the alternative restaurant, Todd English. The latter is $20 for lunch, $30 for dinner. I was taken as a guest for lunch, and I thought it was great. I wouldn't recommend replacing dinner with it, though -- the atmosphere is much better in the Britannia (or Grills, if they're going that way).
Drinks are somewhat reasonably priced. I'm a scotch-and-martini guy on ships, and they run me about $7-9 each, including tip. I would suggest they get the soft drinks package, since it also includes things like mineral water. If you get more than one a day, it pays for itself, and it also makes a non-alcoholic drink more appealing (and thus cheaper) if you just want something to drink in the bar.
Cunard is a bit more "old world," but it definitely has a cachet to it. It attracts groupies, who won't sail on any other line. Part of it is the people -- on both voyages I've been on, I've met people in the bars or at dinner who I'm now great friends with. I know people who have been sailing Cunard since the original Queen Mary in the 1950s. It grows on you.
If they've been on QE2, QM2 is very different, but in a good way. QE2 was irreplaceable, and we're still mourning her loss almost a year later. But QM2 is truly the last great liner, and she has a very special spirit.
I'm 19, approaching the prime target age for a Carnival or Royal Caribbean passenger. But honestly, the atmosphere onboard a Cunarder is much, much more appealing to me.
If there's ANYTHING I can help them with, let me know.