the only obvious way to find out how long your aunt has lived in nepal (without treating your aunt like a terrorist and looking at every possible immigration record databases) is by looking at your aunt's passport for departure date of when she left the U.S.

since your aunt is carrying a nepali passport, have her apply for new passport in ktm because she has now LOST her passport in nepal. new passport won't have any departure dates and Green Card should be sufficient for US immigration officers to let her re-entry without any problem. be sure to have your aunt tell the immigration officer she was only in nepal for 2-4 months. like i said, unless your aunt looks like osama, she won't be asked any further question.

this trick works like a charm!

but next time have her apply for extended stay permit before she decides to go live in nepal for more than 6 months.