You should still be able to efile your return if you (if you are the Primary Taxpayer on the return) get and use an IP PIN. The dependent does not need to get/use an IP PIN for your efiling purpose unless they already have one. You may want to consider getting one for the dependent at some point, however, since you don't know who claimed her (if the Ex insists he did not.) I'm not sure the steps you'd have to take to get one for the dependent, but in terms of you efiling your return right now, the dependent does not need one right now. Only the primary taxpayer on the return would need to use one to be able to efile this return in this circumstance.
Here's what the IRS says:
Starting last year "the IRS will accept Forms 1040, 1040-NR and 1040-SS even if a dependent has already been claimed on a previously filed return as long as the primary taxpayer on the second return includes a valid Identity Protection Personal Identification Number (IP PIN). This change will reduce the time for the agency to receive the tax return and accelerate the issuance of tax refunds for those with duplicate dependent returns. In previous years, the second tax return had to be filed by paper."
"In the scenario where the dependent has already been claimed on another tax return, the IP PIN provides an important new option. The taxpayer listed first on an e-filed tax return claiming dependents can provide their current year IP PIN when they file. If they do, the return will still be accepted. The spouse (if married filing jointly) and the dependents on the tax return don’t need to provide an IP PIN if they don’t have one."
IRS Source:
https://www.irs.gov/newsroom/irs-takes-steps-to-help-prevent-refund-delays-by-accepting-duplicate-de...
