husband sister

Ne💕❣ • Mother To An Angel Baby , Currently Pregnant With Rainbowbaby Coming March 2017
Sorry in advance if this is long , 
Ok , im 22 , my husband sister is 17 .. A couple of months ago she met this guy name At her job recently . They have been talking for i say atleast 3 or so months . Fast forward to her prom , they both had proms but hers was before his , but he had a after party at a family cabin In North georgia ... Because of my husband & I her curfew wasnt 1am anymore it was 6am she had to be home before 8am ....they was drinking doing everything teenagers do , but what she does is she plans without her mother knowing and tells me what shes actually doing but tells me different. Ok so fast forward , 2 month ago she went to a party at his friends house they had sex , after they gotten done he called her another females name . ( they were dating for awhile before this happened ) Mind you she was avirigin , fast forward 1 month ago they kept having sex to which they started having drunk sex , later she came home that day in told me they had sex and he thought he finished off inside her , and her period is late ( her mother does not know she has been having sex with him , she think she is still avirgin ) because her family is christain she is scared that her mother is going to be really mad at her an not talk to her, and put her on restriction example :not to use the car anymore . Anyways recently they have been arguing alot , once they had sex he changed up completely . Today i asked about them she said they are completely over . She hasnt been sleeping and she has been takin pills to sleep , she doesnt eat , she just stays locked up in her room . And she also been crying her little heart out . He was her first relationship . 
But her mom doesnt know why she is so emotional over him, and it was because she lost something special to him  , should i tell her shes been having sex or leave it alone ?
And do you think he was using her for sex or you think he really liked her? I dont want to tell her because it might hurt her even more