1. Nobody can ever give 100% gurantee she'll be fine spending 6+ months outside the US without reentry permit
2. This resets her eligibility for naturalization, as breaks continuous presence test (for USCIS)
3. Her claiming to be non-resident for US tax purposes can bar her from becoming a US citizen in the future. N-400 form has question :
"Since you became a lawful permanent resident, have you called yourself a "nonresident alien" on a Federal, state, or local tax return or decided not to file a tax return because you considered yourself to be a nonresident? "