When I was 17, my friend started a fight with her ex's new girlfriend in a club we were in. She slapped the new girlfriend, who scratched her neck back before bouncers broke them apart. I stayed well away from it all.
When we went outside the new girlfriend went to see her sister, and pointed at us. The Sister, mistaking me for my friend, came up and punched me in the face.
My friend fell out with me because I didn't have enough sympathy for her poor neck, which consisted of three small grazes. She was moaning at me as I was nursing a black eye. She wasn't in the least bit mortified that her violent behaviour led to her friend being punched - in fact she said I should have stuck up for her 
I went round to hers with my mum the next day to get a few things I'd left the previous night. Her parents answered and also had a go at me for not sticking up for her! When I pointed out that I got a black eye because of her violent behaviour they said "well that's hardly Emily's fault that girl got the wrong person".
My mum, in an Uber calm and posh tone said "Gosh, no wonder you've a thug for a daughter with parents like you raising her. Oh well we can't all have wonderful daughters like I do. Have a lovely day" 