See https://docs.aws.amazon.com/redshift/latest/mgmt/data-api.html : "To run a query on a cluster that is owned by another account, the owning account must provide an IAM role that the Data API can assume in the calling account."
You can/should specify and API KEY & ID when you're constructing your client which refers to an identity in the target account.
redshift_client = boto3.client('redshift-data', aws_access_key_id='abc', aws_secret_access_key='123' )
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