I've seen this. The "counter" is to point out that in all likelihood, the person signing the 5500 is a fiduciary anyway, by virtue of function, and then to educate them on being a "good fiduciary." Someone once said "It's not bad to be a fiduciary, it's just bad to be a bad fiduciary."
Besides, hiring a fiduciary is itself a fiduciary function and may expose you to co-fiduciary liability - or at least a duty to monitor....
A deferral election can only apply to compensation that is not currently available when the election is made. See 1.401(k)-1(a)(3)(iii)(A). When did he make the election to defer for 2017? If he didn't decide to defer until 2018, it's too late for 2017 deferrals.
If the person is an owner of certain types of businesses who receives a Schedule K-1 rather than a W-2, he or she won't know the amount of income available from which to make deferrals until some time into the next year. In that case, they would have more time to make a contribution. But since you mentioned an amended W-2, that doesn't seem to apply in this case.
To be a 2017 contribution, it must come from a 2017 paycheck. Since it is not possible to issue such a check (legitimately) after 12/31/2017, the request cannot be honored.
I like this statement made at the 2018 Enrolled Actuaries Meeting: "you can have land in your plan, or you can have me as your actuary, but you can't have both."
Consider the proper understanding of "freezing" an account. It's possible the account was "frozen", in the sense that no one else (ie, other than the person whose name is on the account) has access. The term "freeze" will (probably) not include an action that transfers the invested amounts into another type of investment. In other words, the amount remains invested, which means it can go up or down.