For me I guess it depends on the level of borrowing. Did #2 use the idea like a prompt and have it spark a similar idea? If so then credit would be nice but is not absolutely required.

On the other hand, if #1 outlined a detailed plot or a very unique and original idea (I'm thinking of some of the stuff yomiminamino's done with his novel for an example here) and said outline was used without permission, that would cross a line.

So without knowing the specifics, it's hard to say which side of the ethical line in this particular instance.