An “X” can act as a valid signature on legal documents. The real issue here is whether the person, despite the disease and the treatment, has the legal mental competency
required to finalize the power of attorney.
Competency can be sort of a fuzzy issue—one of those many “gray areas” in the law. When asked to define it, one court recently wrote: "The test of competency to contract is whether the powers of a person’s mind have been so affected as to destroy the ability to understand the nature of the act in which he is engaged, its scope and effect or its nature and consequences."
When powers of attorney are challenged in court, the judges will usually turn to a medical opinion for help in each particular case. If you believe the person you write about has the necessary capacity—and that would be required when he or she signed the power of attorney—then it might be a good idea to get a doctor to sign a simple statement summarizing that belief.