I don't think there is an issue on the discount. Despite the fact that her neighbours are reported to claim she sublet to her brother and wasn't living there.
She was on GMB this morning and categorically stated that she'd had expert advice and that she was living in her ex-council house up until the point she sold it, despite being married. Unfortunately she either didn't disclose the marital situation to her expert or the expert failed to take account of the impact.
The fact remains - married couples can only claim one residence as home, irrelevant of where they claim they lived. Either his house or her house is/was subject to CGT on sale. What is unclear, is if the former couple have filed for divorce and if this has been sealed. The tax rules for divorcing couples changed recently. She will have liability on his home, given her claim that her former home was sold as her primary residence.