My sister is 15 & had sex with someone that isn't her boyfriend!

So my sister told me herself, and of course I'm not going to tell our parents b/c I don't want to lose her trust. 
​She told me she did it with some guy she is "FRIENDS" with that is 2 years older than her. And of course, guys that age only want sex. 
​she's on birth control and everything. 
I know what's done is done, but it just bothers me that she didn't think this through or even come to me for advice before she lost her virginity to some guy that doesn't even care about her. What should I do? 

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