My sister changed...wants a divorce..help, what should I do?

So my sister was happily married with her husband for six years. She got married when she was 18, moved out and always talked about how great her husband is. She would always talk about him, want to be around him 24/7 etc. They had their normal little fights but nothing serious. My sister has her values and would never cheat or "party" (get high, drunk, clubbing etc). I know this because she would always tell me to never do things like that. Two weeks ago she went to this school program in the west, her husband had no problem with her leaving, he supports her education all the way. After two weeks she told her husband she kissed another guy. Her husband forgived her and said he wanted them to both move on. The next day she called and said she wanted a divorce, she had no "real" reason for it though. She said because she wants to continue her education, but he has always been there for her, told her to follow her dreams etc. After that they haven't spoken. My sister hasn't even told me anything or any of her family. It was her husband that told me everything. She just posted on facebook she was separated and deleted her husband as a friend. Now she is talking to all her old friends from high-school (whom she would always talk crap about), I see pictures of her partying and without her wedding ring. Her husband told me to just pretend like I don't know anything. The last time I talked to her, she sounded so different, she was using weird words and talking all fake. How can someone change so dramatically fast? What should I do? What should my brother-in-law do? It hurts to know that she hasn't told me nor our parents yet I see on her facebook that her friends know. I've just been supporting her all the way, she trusts me-atleast that's what she says yet she hasn't told me.