Congrats to your DH for his first Du!!!! Sorry he got the penalty- but that's not bad for his first race. I raced with someone a couple years ago that was awarded 3 penalties for doing 3 different things wrong (we learn from our mistakes, or so I always tell myself).
I was so scared of breaking rules I didn't know about when I did my first multisport event two years ago. I found someone before the race that looked friendly and not super-stressed and asked them to give me a run-down of the biggest do's and don'ts. There's a lot to know, but it boils down to a few major things:
*don't draft
*if someone passes you, it's your responsibility to get out of their draft zone- not the other way around
*you have only 15 seconds to pass someone, and once you start you must pass them- you can't fall back behind
*mount your bike after the line coming out of transition, and dismount before the line coming back in
*buckle your helmet before you touch your bike, and rack it before you unbuckle it

I also learned that it's OK to run with your helmet and gloves on, but you can't throw it down or it's abandonment of equipment.