Note that if you are not the biological father, you can't claim the child. If your GF has more than $4,050 of taxable income in 2017, then you can't claim her. If the GF is older than you, then you can't claim her.
First, your GF needs to file an amended tax return. Since amended tax returns can not be e-filed (the IRS says so) she will have to mail it to the IRS. Amended returns take 12-16 weeks to process.
Next, since your GF already e-filed and her e-filed return was accepted, that means the dependent she claimed has been marked as "claimed" in the IRS computers. Her filing an amended return will not change that. Therefore you would not be able to e-file your 2017 tax return if you claim either or both as your dependent. You would have no choice but to print, sign and mail it to the IRS.
