Let us leave aside the broader question of is it OK in general to stay in touch with friends from before you were religious and focus on the specific question of the original poster and the specific friend.
You have from what you have said known this person for many years. And therefore we assume have some in site into who she really is. Do you feel that she has or may try to do something to turn you away from the path you are following or otherwise harm you? Or has she been supportive of your choices in life? If she has been supportive keep being friends.
Show her the same level of respect for her life as you would like to see shown for yours and I think you will be fine.
This is just friendly advice from a fellow BT, take it for what you think it is worth.