Final answer:
You demonstrate agreement with your brother by endorsing his perspective, which means showing approval or support for his viewpoint on the mutual friend's behavior.
Step-by-step explanation:
You show your brother that you agree with him by endorsing his perspective.
Endorsing is the act of showing approval or support for someone or something. In this context, if you endorse your brother's perspective, you are expressing agreement and validating his feelings or observations about the mutual friend.
This is different from dialoguing, which is having a conversation to reach an understanding, or contesting, which means to argue against or dispute something. Disconfirming would imply denying the validity of his perspective, and orienting typically involves familiarizing someone with a new situation or environment, which is not applicable here.
To show endorsement, one might say things like "I understand where you're coming from," or "I agree with your point of view," providing a meaningful recognition of the brother's frustrations.