I agree that the girls need to be encouraged to have strong boundaries because there are some unpleasant men in the world (and vice versa but responding to this situation). And they did well to tell you and not keep it 'secret'.
I do it to my DD because of my experiences.
But the boys DO need telling this is unacceptable! If by any chance it was completely innocent, they need to be made aware of how their actions make other people feel, or they will grow up unable to make that judgement. And it is doing them a disservice not to admonish them get away with it.
If we don't steer our young people, of course the girls will grow up having to deal with crap like this. Society only changes if people put in the work. Given the tide of unhappiness of girls/women as a result of mistreatment by boys/men that is evident, then making changes, however slight and gentle, might change the situation.
OP I personally would approach the parents and acknowledge how difficult it is given your friendship, but state that the incident had happened and you would like to discuss how to resolve it.
I would try not to get emotional but be practical and state 'facts' so that you are being objective and non accusatory. How they respond, as another poster said, is a measure of the friendship.