If someone is married with a kid and files taxes separately, and claims the child, what happens to both parties?

[meowtwix]

They are still legally married but don't live together but not legally separated, the child is at both houses about 50/50, the husband went to file taxes and it showing he owes a lot of money now

[CatinaT1]

Generally, filing jointly will give you a bigger refund or less taxes due. When you file separately, your tax rate is higher and you won't be able to claim:  

• Education benefits  
• Earned Income Credit (EIC)  
• Child and Dependent Care Credit (usually)  
• Adoption Credit (usually)  
• The same benefit married filing jointly couples get for itemized deductions, the Child Tax Credit, and capital losses (all of these deductions are reduced by half)  
• The standard deduction if your spouse is claiming itemized deductions  
• On top of that, if you live in the community property states of Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, or Wisconsin, you have to deal with community property allocations and adjustments, which adds extra work and complexity to your tax preparation chores.    

Want to compare filing jointly vs. separately?  

Why would I want to file separately?  

[Critter-3]

The party that filed already can amend their return to file joint with the other parent if they can agree to do so ... just because you don't live together doesn't mean you can't file together.  

Going forward  :  one or both of you may need to adjust your withholdings so that you are not taken by surprise again in the future.  From the IRS : 

 There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent** is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner. 

Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the exemption to him.

 So, it's good idea to let the other parent know that you will be claiming those items, as many first time divorced parents are not aware of this rule and may try to claim those items, which will cause the IRS to send out letters.

Ref: https://www.irs.gov/publications/p17#en_US_2017_publink1000170897

Scroll down to "Children of divorced or separated parents (or parents who live apart)"

** The IRS goes by physical custody, not legal custody, for who is the custodial parent.  Furthermore, for tax purposes, there is no such thing as joint custody, regardless of what your legal agreement says. The requirement, to be custodial parent, is that the child live with you MORE than 50% of the time. One of you has to be the custodial parent and the other the non-custodial parent. The IRS goes by physical custody, not legal custody.