From what you've shown of the bounce back, it looks like the sender's IP address is in an RBL (Realtime Blackhole List) which is a list of IP addresses that has been identified as sending spam email. This doesn't mean that your friend is sending spam, but some of his neighbor's on his email server may be. The sender can review the status of his mail sever IP and even request removal from an RBL using this tool:
He may need to get his web host involved as sometimes that can speed up the removal of an IP from an RBL.
Please let us know if you have any questions or need any additional assistance.
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