Dependency on Tax Return Issue

Ok, so for any of you out there who may have gone through custody/dependency disputes, or know someone who has, here's the situation
The past 2 years Mom has claimed my boyfriends son (without discussing it first with dad) on her taxes and he let it go. Since their son was born he has had him over 50% of the time, according to the IRS that means he has the right to claim his son. This year, we called the IRS to double check. They ran through the numbers, or whatever it was they did to figure out who could claim the child and said that he could claim him being that his son lived with him more than half of the time over the past year. So he tells mom that he claimed him, and spoke to the IRS and she said she already filed but would re-file since it was his right to claim him.
So we get the tax return, and he sent her a message asking if she re-filed yet and she said
"I got my money before I agreed to re-file and already spent it".
Now we know if he spends it too, which we really do need the money, that he will be audited by the IRS down the line. His son is 3 and does not go to school because I homeschool him, so there are no school records. We can get doctor records that state that his address is here, but what else could he do to prove that he lived here?
It isnt in writing anywhere in their custody agreement that either of them has more right to file, or that it would or would not be split up, but everyone i've ever known that has joint custody has gone every other year to claim their children. I don't think it's right that she gets the return 3 years in a row when the majority of the past year (actually 3 years) not only was he living with his father, but his father was being made to pay child support (I don't know why) and pays for his insurance as well as all his doctor visits, and for all the classes we have enrolled him in (things at the community center in town like art class, gymnastics, playgroup).
Any advice would be so greatly appreciated since an audit is inevitable and he wants to make sure he can prove it to the IRS in court that he had his son more than half of the time. She is good at manipulating the court, she has done it a few times already regarding their custody arrangement (you may have seen some earlier posts of mine that this has been an ongoing struggle). So we need solid 100% proof that she can't make and win claims against.
**EDIT**
Originally they it was joint custody, yet named her primary although dad had him sunday through thursday, 4 overnights. In December, she wanted to move out of state and when he fought for primary they told him it didn't exist (which I still don't get) and the hours changed at that point to be 50/50. So this coming year will be split in half, but the past 3 years he had his son almost 60% of the time. We ran the return before filing, both with and without claiming him. It is a difference of almost $5,000. There has been a huge on-going custody dispute, as I've stated. He has already claimed him, already received the money. She claims to have spent hers, but like i said she did agree to re-file, and an IRS agent told us that it is his legal right this year, and not hers to claim him. He is already planning to fight this, so I really just want to know what kind of documentation I need. He has, like i said, Dr records that put him living at our address. His insurance is under his dad, at our address. I also have receipts from the past year for food and clothing. Just want to know what else he might need.
she is not the main custodian. it is joint. the court told them that there is NO primary custodian in their case. just because she is the mother it does not automatically make it her right. he is the main caretaker when it comes down to it. anyone who wants to respond that its her right as the mother, please don't respond. it is as much his right as it is hers, if not more-so his. As far as it being a who filed first thing, she didn't discuss this at all with him. She refused to give him their son's social security number for weeks. I'm not sure why he didn't have it in the first place, but she won't let him have the birth certificate or the social security card even though he's here more than he is there and will be going to school here with us. I'm assuming she only gave it to him after she filed. She refused to give it to him last year too so that's why he ended up not filing. When the IRS agent told us this year that it was his right and he should file, thats when she gave him the number and said she would refile. Now, weeks later, after he received his return she says she already spent the money.
I'm going to get all the documents together and hope for the best if they audit.