Well if she entered illegally it is problematic as she is not eligible for AOS. The alternative is Consular Processing which requires leaving the US, but she will trigger a 10-year ban on leaving the US after accruing at least 1 year of "unlawful presence". So she should apply for a waiver, which requires showing "extreme hardship" to her USC or LPR spouse or parent. He has a USC spouse so she should try to find evidence of "extreme hardship" to him if she can't be in the US. Applying for the waiver outside the US is risky because if she is denied she is stuck outside the US. Right now it's possible to apply for a "provisional waiver" with I-601A before leaving the US. She should collect evidence for "extreme hardship" and apply for a provisional waiver ASAP as we don't know whether the provisional waiver process will be ended at some point.