As a stepmom to two who are now in their 20s, whose bio-mother punished them in many ways (including not talking to them for a year a time) for their percieved mistakes or faults, I would agree that you need to just support your son emotionally through this and likely more events like this in the future. Reassure him that her anger and choice to "punish him" had nothing to do with his action/inaction but was a choice SHE made.
It can be painful to watch a young person come to accept that their parent is who she is, but he will adjust to that over the years, especially with two supportive parents in the picture.
It took me years to fully realize that we could ask, request, demand, mediate,or take her to court, but we couldn't make her do or not do anything. Put your energy and love into your son and all will benefit. And bless you for being a caring stepmom with all its challenges and rewards!