It is up to your group therapist to deal with the angry fan thrower. You do not need to feel pressured to "forgive" her.

I would explain in no uncertain terms to the DBT group leader that you no longer feel safe and what does he/she plan to do about it.

They also have a consultation team to which they must report each week, so if you can't get the group leader to act, then report directly to their consultation team.
Best,
Okie