bye
http://momondo.com (http://momondo.com) is a very good site, and it's an easy way to visualize all available flights, dates, etc. Some consider http://matrix.itasoftware.com/ (http://matrix.itasoftware.com/) to be a better site. Best wishes! I really hope you can find a flight for your friend.
Kayak.com, I think it searches all of the travel sites for you.
+1 for kayak (I planned miami using them)
that said, there are deals from time to time at http://www.cheapoair.com/ (http://www.cheapoair.com/) (when comparing, be sure to factor in taxes/fees, cheapo leaves them out on the comparison matrix/results page).
Check http://www.kayak.com (http://www.kayak.com) and http://www.lastminutetravel.com (http://www.lastminutetravel.com) to see which airlines are offering the cheapest flights. Then buy the tickets through the airline's site.
For a while I had to fly almost weekly, and I usually used travelocity; with some work, they can give you a good deal. I didn't really shop around though, so I can't speak for its quality as compared to other sites.