Top-Heavy for HCE in combo plan?

Well once you make any employer contribution to the DC plan that is out side safe harbor rules you blow your "get out of top-heavy free card" so he needs to get a top-heavy minimum since you are clearly going to have to make an employer contribution to make the CB plan work. Though you can probably limit the HCE to a 3% Th minimum in the DC plan if you exclude him from the CB plan and your TH coordinating language is right.

If they are trying to go with lowest cost they might consider switching from SH match to SH non-elective next for next year.

If they put in a new PS plan, they can keep the TH exemption for the existing plan as long as it continues to consist solely of deferrals + SH match. Then then can exclude this particular individual from both the CB and PS plans, and they won't need to give them a TH minimum.

If that's too much trouble, they could exclude the individual from the existing DC plan going forward; they would probably still need to give them a 3% TH minimum for 2021 (assuming they are excluded from the CB plan), but after that, nothing.

The key, obviously, is to make the HCE actually ineligible by job title or other mechanism, as opposed to having him be eligible but just in a group accruing $0 benefits.

Alternatively, the HCE could waive participation in the CB plan and continue to elect 0% deferrals in the 401(k).

Generally you can't waive participation in a DB plan - it is not an elective contribution.

If you mean the irrevocable waiver of participation, that has to be done before the employee becomes eligible for any plan offered by the employer, and must apply to every plan ever sponsored by the employer. If they are already eligible for the 401(k) plan then it is too late. That's even if the plan allows irrevocable waivers, which plans are not required to offer.