Hi,
I think you are confusing 2 forms. The FBAR (Report of Foreign Bank and Financial Accounts), which is mailed separately from the tax return, is required if at any time during the year the U.S. person (your wife in this case) had bank accounts or investment accounts worth at least $10,000. So that's where the $10,000 figure comes from.
I am not aware of any limit below which a USC does not have to file a tax return, but someone will correct me if I am wrong. All US persons regardless of where they live in the world and regardless of where their income is earned must file a US tax return every year.
If she is behind on filing for those years, she will need to catch up before she is able to renew her US passport, if not before.