You never stated it, but I am assuming you two are not married but share a biological child.
The IRS has tie breaker rules when two parents can not agree on who is to claim the child
- Tiebreaker #1: parent who the child lived with the longest during the tax year
- Tiebreaker #2: parent who earned the most money during the tax year
There is an argument to be made that your son was only temporarily away from home (visiting grandparents) so the fact that he was with Mom on those nights doesn't change the math. If you are saying you all lived in an apartment for 9 months and then you all lived in a house that was being renovated and Mom / Son spent a few nights at Grandparents during renovation, then child spent 365 days with both of you and Tiebreaker #2 is the imperative.
What I am stating is that if you all lived in a home for the entire year and let's say Mom took Child on a vacation without you doesn't mean that Child spent more nights with Mom. Child was only temporarily away from home and still spent 365 days with both of you.
Who paid more than 50% of the household expenses? if you did, she can't file HOH.
It may be worth sitting down and comparing the tax returns both ways (I am assuming that with that larger income, you pay more than 50% of the household costs)
1) she files SINGLE and claims the child and you file SINGLE.
2) you file HOH and claim the child and she files SINGLE
I suspect you will find that 2) is the better outcome - by far. Better to agree who is to claim the child than fighting and letting the IRS figure this out.
To prove it to HER, here is the IRS reference to what a 'temporary absence' is:
Temporary absences. You and your qualifying person are considered to live together even if one or both of you are temporarily absent from your home due to special circumstances such as illness, education, business, vacation, military service, or detention in a juvenile facility. It must be reasonable to assume the absent person will return to the home after the temporary absence. You must continue to keep up the home during the absence.
page 9 middle of the page
https://www.irs.gov/pub/irs-pdf/p501.pdf
