1) She could apply based on marriage to US citizen, as long as you were a citizen the 3 years, she was married to you for 3 years and you lived together for 3 years. Move evidence needed for this type of filing, but then her 6+ months trip will be outside of 3 years lookback
2) She could apply 4 years and 1 day since returning from that 6 months trip under 5 year rule
There's special rule about that:
https://www.uscis.gov/policy-manual/volume-12-part-d-chapter-3
3) She could wait until 5 years anniversary of coming back from last 6 months trip, then she wouldn't have to list it
4) She can apply whenever eligible even with 6 months trip and try to argue she maintained residence. It's not about why she left, but what all ties to the US she maintained. Lease or mortgage would have been a strong indication.