I have always stressed this point. Having lived in the US for a long time and/or having a US degree of any kind does not automatically make a convincing case. Buffalo wants evidence for specific aspects of one’s language proficiency as indicated in the instructions, such as training in English, use of English, etc. What’s more important is you have to have hard evidence to support your claims. Having lived in the US and/or having a US degree itself is not sufficient evidence. You have to prove them.
I never had to get into this kind of situation. I have similar education background (time-wise) with your friend, and I have worked for various agencies where communication skills are most crucial, but I still wrote a 3-page letter and provided 20 pages of supporting documents.
As for if you should resubmit the letter with new evidence or just take the test, I really don’t know. But I wish him the best either way.