Regarding what you can do, I think everyone has a shared responsibility regarding the calling out of sexist behavior.
If you have a friend who says something that is sexist, call him out.
Regarding peoples frustrations it makes me wonder why people are so ready to jump to arms if the perpetrator isn't white?
I mean women have been telling us for a long time about how they suffer from sexual violence but none take heed. Until the perpetrator is brown.
Statistically speaking most rapes occur in someones how with someone they know.
I can't imagine the frustration women must feel when talking about the experiences only to be neglected until shit like this.
Luckily I don't have friends who display this behavior. If they did, I wouldn't call them friends anymore also after calling them out.
The problem is, if you are brought up in an environment that is OK with this kind of behavior, nobody is calling them out. And then it is very difficult for outsiders (teachers, police, etc) to break that behavior. How do you educate someone who has all their life been told women are less then men and who are not punished when displaying this behavior? That's not a simple thing to do. And how do you then convince people to take in these people and try anyway?
I don't know why people are quicker up in arms when it is someone brown or black. It has racists roots most probably, fear of something strange, I won't deny that. It's wrong of course. But if basically 80% of police requests for information about criminals on the local news and local newspaper sites are about immigrant youth year after year with no improvement, how do you explain that to people? There needs to be a line of improvement - and communication about that - to reassure people things are being handled. Otherwise people will get upset if immigration from this groups goes up, like we have seen lately with the refugee crisis.
I won't go around blaming everyone from immigrant descent. But I will question the way these issues are handled by police, politicians and parents.
I'm ranting a bit, because I don't know a quick solution to this also. There really isn't one I think. But meanwhile, society gets rougher and the gaps between groups are growing, between locals and immigrants, people and politics, victims and police, and it's a sad thing to witness.
I'm talking wider crime here btw, not just sexual assault like the incidents in Sweden now.
Edit: When talking about sexual assault here in the Netherlands, immigrant groups are very over represented btw. People with (Northern) African and Middle-eastern background are 3 times as much suspect in these crimes as locals. That was for 2005 (
http://nl.aup.nl/wosmedia/296/vol_83_no_2_-_zedendelinquentie_en_etniciteit.pdf - Dutch research) so the actual worries about this are not that unfounded.