I addition you wrote:
... The title he used above the key was underlined Bitrexx Secret Key...
According title wording I would assume these are API keys. But it's weird they are single strings (these should come in pairs) and 16 chars is quite short ... so I guess this is not a valid point.
last desperate hints:
Maybe your uncle mistakenly swapped those two backup codes. Try using 6 digit code generated from bittrex backup to login into coinbase.
In past I had 2fa sync issues on my old device. Couple times worked using the expired 6 digit code. Try writing down current 6 digits, wait till new set of digits is generated and try to login using the old ones.