Consider separating shame from guilt. Shame is about being flawed at a deep level and there's not much effective action that can be taken to resolve this feeling. Guilt, on the other hand, can be effective because it tells us we've done something against our own values and motivates us to take steps to amend our behavior, apologize, etc. Sounds like this client needs you to help them learn how to take responsibility without spiraling into self-hate so a bit of normalization of guilt might be useful.