Well, that'll surely reduce the used bike selection. Meanwhile, his $650 budget will be more or less sufficient for casual and/or entry level racing. If he just wants a CX bike for general use then $650 will be more than enough imo.
If he's serious about getting out on a CX course he might still want to hold out for a high quality used CX at a good price. However, purchasing a new and properly sized NB CX1 would, of course, put him in the saddle right away.
Btw, your friend might also consider renting a few (CX bikes) just to get a feel for them before he buys (some race events do offer bike rentals).