If a S-Corp is a sole owner/shareholder corporation, can a POA for that person apply a POA to the S-Corp?

I would say not... the POA acts on behalf of an individual, not a corporation. I am a signer on several of the bank's accounts - my POA is not authorized to sign business check on my behalf. I wouldn't disinguish between a single owner or multiple owner; your customer elected to incorporate rather than operate as a sole proprietorship.

Just add the person as an authorized signer and grant them whatever powers they need on the resolution.

A corporation is a legal person separate and distinguishable from its owner(s). It may appoint agents, including an attorney-in-fact. But an attorney-in-fact for an owner of a corporation may not act as the attorney-in-fact for the corporation unless the corporation has separately granted him or her its power of attorney.

Similarly, an agent of the corporation may not act as an agent for the corporation's owner, unless the owner himself or herself separately appoints that individual as his/her agent.

It doesn't matter whether the corporation is an S-Corp. or PC or any other variant of a legal entity, including an LLC.