Many experts would consider a 6 week negative to be conclusive.
There is a logic behind that, the average time to sero-convert is 21/22 days. Body starts producing antibodies to HIV immediately after infection but even after sero-conversion though the body is already producing the antibodies, the detectable amount of antibodies by the test may be or may not be generated right away. It usually gets detected approximately 7 days post sero-conversion i.e 28 days.
Hence, to be sure with a buffer of 3 weeks from sero-conversion process - A 6 week test is recommended.
In your friend's case, it really doesn't matter if he tested at the 83/84/85 days, as he tested well after the aforesaid period. Even if I go with 3 months or 12 weeks standard as suggested by some experts - A day or two makes no difference really. He's in the clear.