If he has proof of onward non-Schegen travel - he’ll be ok, which it sounds like they accepted (DOCS OK). He might have to explain again in SEA, but unlikely if it says DOCS OK. If traveling on a US passport, the likelihood of anyone caring about his return arrangements in CDG is infinitesimaly close to zero - should have no problems.

Keep proof for the return (preferably in the form of Schengen exit stamp on the way to Croatia).