A man will tend to play down the incident or tough it out, often making a joke of it. Take action with the first slap. Don't be melodramatic or wait until things have started to cool down. It's important to act decisively. Look for reasons for her behaviour.
Research shows that domestic violence is often the product of a violent upbringing.
Explore all these avenues, decisively and precisely, and then let it rest. But let her know that the first slap was taken very seriously indeed. If it happens again, there is a risk of a pattern being established and even more decisive action must be taken. If you spot a trend appearing, make sure you discuss it. To ensure that she knows how seriously you view the second incident, it may be time to consult her family.