PAL and NTSC is against the DVD specs and there are few, if any, DVD apps that will produce such a disc. The only way I know of to do this is to cheat -- ie, use a program like DVDPatcher to change the header info of one of the files so that the DVD app thinks it is the other format.
And you're sure that the client didn't just say / mean that the DVD was multi-region or region free? Regions (0, 1, 2, etc) are not the same animal as formats (PAL, NTSC, SECAM)?