Defer statements run exactly at the moment of a function return (in lifo order). They are equivalent to wrapping the rest of the function body in a try / finally. They have to, since you can change the return value in a defer (by using named return values).

A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

https://golang.org/ref/spec#Defer_statements

There might have been something else going on with your friend's code. Maybe the function he was using didn't return before running out of file descriptors?