I think that eventually she will get past taking sides. Especially if you arent putting her in a position where feels like she has to defend her son, I just try to think how would you react if your son had done this? While you would be very disappointed, and would probably tell him so, you probably would feel like you had to defend him just a little.
I think that you are now (and have been all along) doing a great job, and you know it! Dont let her smack talking get you down!
I guess I gave the wrong finger to the wrong man...