Since you don't specify the kind of crossbow that will be used, the answer must be relatively general.
Crossbow arrows are shorter and heavier than those used for vertical bows so they don't glide as well and are more easily pulled off course by large vanes or heads. What works best for me (in three crossbows) is an arrow with a relatively high front of center ratio (Google FOC).
Personally I use carbon arrows (they are more durable), specifically: Gold Tip Lazer II's, with a 110 gr brass insert and 2" Blazer for vanes. Using a 100 gr head, these arrows weigh 405 grns and have a FOC over 18%. They fly very consistently for me.
For broadheads, I prefer Slick Trick magnums, either 100 or 125 gr depending on which bow I'm using.
The minimum weight and kind of nock your father will need to use depend on the specifications for his bow.