The way I would approach this is to simply inform the participant that the money is no longer in the plan (and therefore not invested), the distribution is taxable and has been reported to the IRS (so that if his tax return does not reflect it, he risks the IRS crawling up his ...), and if he doesn't cash the checks (which are no long plan assets), he risks them being escheated to the state - which causes him to risk losing the money (depending on the unclaimed funds statute in the state he resides in).
Bottom line, it's not the plan or the employers problem once the check is properly issued and delivered.
And by the way, if this were my employee (and an HCE to boot) I would question why I had someone making a boat load of money on my payroll who is so STUPID.