I would politely tell him that he has misunderstood, and that while 3.5" USB drives are widely and cheaply available, 5.25" USB drives are not. You could also tell him (unless you already have one) that you're planning on purchasing and using an FC5025 as a USB bridge to connect to the drive to any machine (since that's what you probably want to do in any case).
And if he tries to send you a link to one of those "combo" units a multicard reader, 3.5", and 5.25" drive in it, you can tell him that they do not, in fact, run the 5.25" floppy through the USB interface, but have a separate 34-pin floppy connector on the back (meaning you'd need the FC5025 anyway). Also, they generally have really crappy compared to quality TEAC or other drives.
Kam