Whew. I didn't think this was going to generate so much discussion like this. I will try to address everything, but if I miss something, please let me know.
DS is a flexitarian, it's just easier to say vegetarian because people know what it means. This is a good explanation: http://vegetarian.about.com/od/glossary/f/flexitarian.htm
It was her compromise with the free range, organic turkey from a farm within a 100 mile proximity. Like I said, she is a flexitarian for environmental reasons, so eating mass produced meat is against in what she believes; free range organic, is not. When it comes to the vegetables in other peoples homes, she couldn't care less where from they came (mass produced vs organic), but she tries to buy as much organic, locally grown stuff for herself as she can. DM said, "You will eat turkey!" so DS said, "Okay, but only if!" DS was trying to compromise with mom in a way that was acceptable to both of them, not one appeasing the other.
DM has had other invitations for Thanksgiving, so if she doesn't cook for DS and BF, she would be going to someone else's house.
The turkey is certainly a red herring. DM likes to have control, and when she starts to lose is (ie: this month being particularly difficult), she seems to clamp down harder.
This is definitely a precedent setting issue; I do not see any outcome where DM would not down the road say, "Yeah, but you ate that turkey at Thanksgiving!" because that's just how DM is.
I think the best bet is to get DS to ensure that she gets the organic, free range turkey, to put that onus on her so that everything is a compromise. "DM, like I said, I will only eat the turkey if it's free range, organic, so I've gone to the trouble of getting it so that you don't have to worry about it. If you don't want that, maybe we can get you just a turkey breast so that you're not cooking an entire turkey for just one."
Thanks, everyone, for trying to help me find a happy medium for the both of them. So far, DS is the only one that has called me. DM doesn't talk to anyone when she's upset.