I’m not the best judge of this, but he is entitled to claim the benefits if he is unemployed. If they ever review his claim, after getting benefits, he might be asked to repay. But since he is out of “that work” no reason why he shouldn’t be able to go ahead claim on that job at this time. Not sure, but I don’t think the claim asks about “other employment” or income such as retirement benefits. And since the state of Illinois is basically shut down, I couldn’t find an expert to ask! So give it a try!
And here is a link to the state unemployment website, assuming you don’t have it.