"That's true if and only if he didn't continue to refine and contribute to the code after entering employment, and specifically, didn't use it at his place of employment."
The same could be applied in reverse: if he wrote a considerable part of the code before entering employment and did use it at his place of employment, what does it say about the employer's right to use it? Or is it "since you're working here now, we can use all you ever wrote exclusively even if you didn't license it to us"?
Without a specific licence there would be an implicit contract that the employer could use the library for any projects that Nick has used the library on.
If after Nick leaves they would be on shaky ground if they used it on other projects that Nick hadn't worked on.
IANL but that's my understanding of the law in the UK.
The same could be applied in reverse: if he wrote a considerable part of the code before entering employment and did use it at his place of employment, what does it say about the employer's right to use it? Or is it "since you're working here now, we can use all you ever wrote exclusively even if you didn't license it to us"?