How serious would he be? If she thinks he'd really get into it, good ones are Fisher
http://www.fisherlab.com/, White
http://www.whiteselectronics.com/, Garrett
http://www.garrett.com/. This company
http://www.kellycodetectors.com/indexmain.htm is good to deal with, bought my fisher there a few years back, just wish I had more time to play.
Unless he's more blow than show, I'd stay away from the one's from Sam's, Wal-Mart, Radio Shack. A good md will set you back a few C notes, but they are quality instruments made to last several years. The ones sold at the above places are basically toys, IMO. Entry level (110 film cameras, so to speak! ;) ) items made for folks to say they have a metal detector. Tell her to read through that kellyco site, lot's of interesting things available now days.