She's toxic alright. Probably the best thing is to ignore her. Even without knowing who she is referring to, it's obvious to the group that she is being immature about something. I don't think it really matters whether you tell the workshop leader, or not. He can't solve your problem for you. The less reaction she gets from you, the sooner she'll give up this nonsense. She wants to upset you. Don't let her. Be glad you now know what kind of a person she is. She's childish and probably not too happy with her life. No one is going to base their opinion of you on what she says. Try to get friendly with a few other people in the group.