bye

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/ 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/ (when comparing, be sure to factor in taxes/fees, cheapo leaves them out on the comparison matrix/results page).

Check http://www.kayak.com and 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.