A lot of the relevant facts are missing; however, as a general matter, unless your mother qualified as "Head of Household" for your son (i.e., in very general terms, she provided for more than 50% of his support - there are other tests, but this is the gist of the test), your mother cannot claim your son as her dependent, regardless of whether you authorized her to or not.
If your mother did not qualify as Head of Household for your son, then IRS has most likely disallowed the deduction and, at least for her 2011 tax return, IRS will most likely be sending her a letter stating that they have disallowed the dependent claim for your son and asking her to either agree with the disallowance or provide proof that she is entitled to claim your son as a dependent.
If your son's father also claimed your son as a dependent on his own tax returns, then IRS may be assessing his return and your mother's return to figure out if anything funny is going on. In this case, it might take a little longer, but your mother would receive the same sort of letter I described above.
A simple way for your mother to start trying to get a handle on where her refund might be is for her to go to the IRS website and follow the instructions for the "Where's My Refund" function, which will allow your mother to get some basic information. The IRS webpage for "Where's My Refund" is here: http://www.irs.gov/individuals/article/0,,id=96596,00.html
Beyond that, I would strongly suggest that you and your mother at least consult with a competent tax attorney, CPA, or enrolled agent in your area to see if you can get better guidance from someone who can ask you for all of the relevant information; many of them will provide you with a free initial consultation - you won't get everything resolved for free, but at least you should get a better idea for whether you need to have someone start working with the IRS for you or whether you should wait a little longer before retaining anyone.
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