NTSC-J?

The main difference between NTSC and NTSC-J (0 IRE vs 7.5 IRE) so the footage will look different, but otherwise an NTSC disc will work.

Take a look here for a good article on IRE/Black Levels

http://www.signvideo.com/dv-black-levels-dvd-authoring-mpeg-2-part-1.htm

Assuming your friends in Japan have a DVD player and TV set both produced for the Japanese market, your DVD will look no different.

FYI: The analog black level is different in Japanese NTSC:

"Only Japan's variant "NTSC-J" is slightly different: in Japan, black level and blanking level of the signal are identical (at 0 IRE), as they are in PAL,
while in American NTSC, black level is slightly higher (7.5 IRE) than blanking level. Since the difference is quite small, a slight turn of the brightness knob is all that is required to enjoy the "other" variant of NTSC on any set as it is supposed to be; most watchers might not even notice the difference in the first place."

http://en.wikipedia.org/wiki/NTSC

A made in Japan DVD player will output 0 IRE which matches their TV sets (0 IRE) made for use in Japan.
DVD players which are shipped to North America will have their output factory set to 7.5 IRE to match the TV sets (7.5 IRE) used in North America.
Your DVD will look the same in both countries.

Camcorders are a different story however.
When you hook up your "made in Japan" miniDV camcorder to your TV via the analog composite or component output you get your black level at 0 IRE when you play back your SD miniDV tapes on a TV made for use in North America.
So it looks darker than normal over the air American broadcast NTSC signals.