Steve,
You can do this, but it will most likely degrade the quality.
Try it yourself. Just compress a small selection of the video you want, create a simple DVD in DVDSP2 and burn to a disc, then watch on your TV. If it looks terrible, get it professionally converted. If it looks OK for your client's purposes, then go with it.
PAL DVD players will play NTSC discs. However, the reverse is NOT true. So if you were to go "straight to DVD" with your client's footage, then it would play fine in a PAL player or a computer, but NOT in an NTSC player.
-Jeff