The support issue is secondary. He does not live with his mother and makes more than $4200, so, as you said, he clearly does not qualify as his mom's dependent*.
With the tax law change, effective 2018, most dependents will get the same refund whether they claim themselves or not. The personal exemption has been eliminated and the standard deduction increased. So, there is increased incentive for doing what they're doing. But there's increased incentive for IRS enforcement, as well. The fall back will be mostly on the mom, as the son has nothing to pay back. $1800 sounds like she claimed EIC, and that is a serious error (the other dependent credit is only $500).
*If he moved out of her house in 2019 and was a full time student, or under 19, or if he only lives in his own apartment because he is away at school, the answer could be different. .
