@ChristopherT34 - It depends on who the somebody else is and where they live. A child can be the “qualifying child” dependent of any close relative in the household. If she lives with someone else, e.g. her parents, they can claim your child, ahead of you.
For an unrelated person, the rules are different. They would have to live with the child ALL year and provide more than half his support. That's unlikely from what you describe. Even if they qualify, they don't get the big bucks (no EIC & child tax credit).
You say the mother worked some. That will qualify her for some EIC and maybe some small child tax credit.
I suggest you claim the child and see what happens. Let her know you're doing it, it might scare her off. You will probably have to pay back some of your refund, but may prevent an unrelated person from getting away with tax fraud.
You may have to take her back to court to get a more enforceable order (requiring form 8332)
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If someone else claimed your child inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming the child as appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you'll receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest. The custodial parent almost always wins. The non-custodial parent can only claim the child as a dependent if the custodial parent gives permission (on form 8332) or if it's spelled out in a pre 2009 divorce decree.
References:
https://www.thebalance.com/claiming-same-dependent-audit-risk-3193030
http://taxes.about.com/od/dependents/qt/Dependents-Audits.htm
www.eitc.irs.gov/EITCCentral/f886-h-dep.pdf
