Neither of these offences is going to prevent his being granted a NZ visa, but he can be told to ask for a character waiver. Here are the regulations on that.
http://www.immigration.govt.nz/opsmanual/35058.htm
Asking for a character waiver, if instructed to do so, involves writing a letter acknowledging the offence, apologizing for it, (grovelling, really), giving an account of the circumstances (e.g "I was young then, I was in bad company" etc.), and stating the present-day upright lifestyle so he wouldn't dream of doing anything like that any more, and providing character witnesses such as people of standing in the community who know him personally.
However, this may not be necessary, since the cannabis incident was not a conviction, and the drink-driving did not happen within the last five years. BUT HE SHOULD STILL MENTION BOTH INCIDENTS TO INZ, then he can't ever be accused of fraud.