Just guessing about that particular situation, but...

it seems to me that the attackers may have been counting on the boyfriend not knowing what to do, and that he would be reticent about hitting them.

The usual drill is to do as much as necessary and as little as possible, right? I would assume that the first move would simply be for the boyfriend to put himself physically in the way and yell; second move, pushing or pulling attackers away.
The interruption might give time for the target to run for her car. If the boyfriend was at all imposing, that might be all that was needed. If he was little and skinny, he'd have to put on a good show of intensity, and hope his girlfriend could help. The two targets got away "without getting hurt seriously" even though the boyfriend didn't defend, which suggests to me that the three girls were NOT out to do serious damage. It might not have taken much in the way of violence to get rid of them.

I'm a small and skinny female, and have used intimidation before against aggressive men (and even had it work, possibly because of surprise). Will be interested to hear the guys' opinions.
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God grant me a good sword and no use for it. -- Polish proverb