Even if he say checking 5 times before sending,only one character wrong and this is not identical address.
So, they either entered the Bitcoin address incorrectly (although I'm not even sure what the odds of generating a valid address when mistyping an address are?

)...
According to the
bitcoin wiki the chances of a misstyped address being valid is
1 in 232 or
1 in 4.29 billion.
Although, using the "Signed Message" option, you can start typing an address and it shows the partial matches. ("Settings -> Tools -> Sign Message", click the "Address" box and type the first couple of characters of the address).
OP mentioned the address does not show up under the signing tool

- Does the address show up under 'Options' -> 'Tools' -> 'Sign message' from the drop-down menu?
No