Glad you didn't send the email. You and W already had the conversation boundary talk. Now just enforce it. But I wouldn't tell her you are. Just act uninterested and end the conversation with her mid sentence. Gotta be quick on your toes to catch it.
W:Can you pray for my bff and her....
You:..ok, really busy. Gotta run. (with a huge smile on your face like you are headed to Disneyland or something)

If you shut her down and then tell her why, she will think you are doing it out of spite. When you act indifferent to her or anything in her life that will bug her more.

Me: 46 FWS: 36
Married and Divorced 4/07, Pregnant 7/07,False R 7/07
Baby Girl born 3/08
Kicked him out because OW: 7/08
5/10 He realized what he had and lost.
Moved home! REMARRIED 3/14/11!!