as others have said, miscarriages can happen for a variety of reasons. and, 50% of women miscarry on their first pregnancy anyway, so it pretty much happens to half of the population. I assume that your dad's g/f at the time miscarried her first pregnancy, which is fairly "normal."
but, that doesn't mean that veganism isn't necessarily a concern. a well planned, diverse vegan diet can be healthy for the average person in any stage of life (including pregnancy).
but there are areas that can affect fertility:
1. diet too low in calories: some folks go veg and simply don't eat enough. this can lead to a loss of fertility (a loss of menstruation which means also a loss in ovulation);
2. a diet too low in fat: fat is healthy for the body and helps make healthy cells and healthy hormonal balance;
3. a diet too low in cholesterol: cholesterol is important for thyroid and hormonal health, and while most people make enough on their own, some do not and therefore veganism may not be for them as it is basically cholesterol free;
4. a diet too low in diverse nutrients: many vegan groups give information about nutrient deficiencies such as b12, anemia, etc that can lead to other health problems including negatively affecting one's fertility;
5. a diet with too much soy: soy contains phytoestrogens which, when over consumed or if one is extra sensitive, can negatively affect the fertility cycle for some people, but one can be a soy free vegan if this is an issue.
Veganism isn't necessarily problematic to one's fertility cycle (ovulation, menstruation, and maintaining pregnancy). It can be, but if these things are taken into consideration when planning the diet, it shouldn't be a problem.