Can a person with power of attorney sign over ownership of everything to themselves?

The attorney-in-fact or agent (those are the terms for the person given power by a power of attorney or POA) may well have the legal authority to transfer ownership (if the POA by its terms gave her power to sell his assets generally or this business specifically, she could do this), but must do so only in accordance with the principal's (the person creating the power) instructions or best interests: that is, the agent has a "fiduciary duty" of loyalty to her principal and cannot act disloyal. An action violating fiduciary duty could be reversed or set aside by a court in a lawsuit.
However, that is for the principal (or if he is incapacitated and has a legal guardian, his guardian) to challenge her actions: if you are an employee of the company, you have no legal "standing" or right to challenge this. It is he who owned the business, he who gave her the POA, and so he who legal rights in this case. If he is content with what happened, no one else can challenge it. So unless the owner complains, this will stand. And given that this is his wife, who also works as his accountant, who has taken ownership, and that he is hospitalized, for all you know, this *is* in accordance with is wishes or interests--this may be part of their succession plan to insure continuity of management, for example, in case he is disabled or impaired or worse and cannot run the business. Someone turning over a business to his spouse is not that suspicious or unusual.