Her son will almost surely age out and be ineligible to join her as a derivative. For him to get a green card*, his mother would have to file for him after she gets her own green card, which would take another 7 or 8 years, by which time he would be 36 or 37! And if he gets married, that adds on even more years (worse if he marries before his mother is a US citizen).
*unless he qualifies through another route such as employment or marriage
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.