Q. I'm guessing now she should just file by mail.
A. Yes, it's that simple. Do not attach any explanation, just mail the return with copies of her W-2. If the mother doesn't file an amended return, within the next several weeks, the IRS will send her a letter asking her to amend. If she doesn't amend, after getting that letter, then both the ex and the daughter will get letters asking then to defend their position.
Q. Anybody have any experience on a situation like this?
A. It's very frequent and the answer is the same, for everybody; paper mail your return. The stimulus incentive has led to big increase in cases (mostly young people perfectly content to let their parent claim them in the past, but not now with $1200 on the table). You are correct, turning 24 in 2019 and having more than $4200 income, makes her ineligible to be a dependent. Student status is not relevant at age 24.
Q. I suspect it also fraudulently allowed my ex-spouse to get a higher stimulus check?
A. No. Only children, under 17 qualify for extra stimulus. She did get a $500 other dependent credit if she had a tax liability (non refundable), when she filed her return. Starting in 2018, the other dependent credit replaced the old ~$4000 dependent exemption (deduction).
Q. I suspect it may be months before my daughter can get that $1200 stimulus from the government.
A. Yes; it will most likely be next year.
"In essence, the stimulus check acts as an advance of your 2020 income tax refund. This means when you prepare your 2020 income tax return, there will be a line to include the section 6428 credit. The credit on your 2020 return is subtracted by any amount received as a stimulus check in 2020. If the amount you received as a stimulus check is less than the credit you are due, the difference will be included as part of your 2020 refund. If you have been overpaid by receiving the stimulus check, however, you will not be required to return any excess amount". For more info, see https://ttlc.intuit.com/community/taxes/discussion/re-if-i-removed-my-25-year-old-as-a-dependent-and...
