Tell her the truth. Its not in anyone's best interest to forage a relationship or friendship with people we meet in therapy groups. They can set us back or trigger us causing our own emotional health to regress. Ask her politely to not call again then block her number from calling. If she won't accept what your saying have the group counsellor speak to her privately. She will get the message. It's hard to say no, I get that, I may sound like a cold hearted *****, but I'm not. Your mental and emotional health comes first. She has a dependancy problem in more ways than one maybe and she is not your problem.

The first rule in my dbt class was friendship outside of class was strongly discouraged. When it ended one woman tried to get phone numbers and not one person gave theirs out.

Take care of yourself. You're a nice person but you have permission to not be nice all the time.