If he's considering taking the plunge into fatherhood (or is already a father), most men worry about their ability to: 1) Pass on what they know; 2) Not pass on the bad parts of their parents/fathers/childhood experiences; and 3) Be a balanced/fun/strict/good dad.

How she can help:  As all parents know, there's no rulebook for parenting; it's a tough gig. She can talk to him about her fears (because most women worry about motherhood, even if nurturing comes naturally to them), and then ask him about his. By sharing your similar issues, you can make it safe for him to be vulnerable.