Does she have to wait for response from USCIS first and apply for divorce or apply while it’s pending and call the USCIS to withdraw the case?

A friend of mine is having a huge problem. She’s married with her USC husband for almost 3 years. On their 2 years being married, their marriage is starting to fall apart for the reason of the USC husband doesn’t seem like interested to have a family with her such as live on their own. They’re living with his brother. She wanted to start, save and have a baby together but he refused every time she asked him about it. After that they decided to move along on their lives. They do enter in a good marriage. She wanted to save the marriage. It’s just the husband that doesn’t seem want it. Then she met this guy, started talking about their problems together until they fall to each other and had a baby together while married to her husband. Long story short, during her pregnancy, she asked the husband if there’s any way to save the marriage. He wanted to but his family is controlling him not to work things out with her. They filed jointly on October 2015 and did biometrics on November 2015. She had her baby on December 20, 2015. On December 24, they decided to separate because the tension between her and his family’s husband are getting ugly specially her step-mom. She’s so angry at her and called INS and reported her which is she has no right at all to do so because she’s just a step-mom and shouldn’t put her nose into other’s business. When is it safe for her to file divorce? If ever she will get to remove, will the judge allow her to bring her newborn with her to her place of origin? During the process of i751, is it ok to divorce, wait for any RFE, and if they ask for RFE, can she withdraw the joint application, ask for more time to get the final decree and re apply, will she get approve and prove she entered in good faith marriage?