Effect of power of attorney

We have a client with Altzheimers

Some time ago, his wife set up an enduring power of attorney in preparation for the day when she would be needed to sign documents (ie tax returns) on his behalf.

His condition has deteriorated to the point at which she felt it necessary to execute/activate (or whatever the term is) this power, and she now has the authority to sign his tax returns.

However he continues to have periods of lucidity and apparent full cognitive ability, intersperced with periods of mental incapacity.  During those (lucid) periods he prefers to sign documents in his own name.

Do those signatures have any legal force, or do we have to insist on the wife's signature on tax returns now?

Thanks for any input.

With kind regards

Clint Westwood