If someone has Power of Attorney for Health, Welfare and Finance then that individual acts as though they are the deceased.
Their decision is final.
The Funeral Directors and crematorium staff legally have to uphold that person's wishes.
I tend to go along with Vindec and feel it is in your wife's and the deceased's best interests to let it go.
The Power of Attorney was gifted by the deceased and gifted for a reason. It's not easy to obtain. Therefore, as unpalatable as it may be, your wife has to accept the situation.
If somehow you do go to the funeral, accept that it may be your wife and your presence (and any other people excluded) that may result in any potential conflict, which is surely not what either of you would want.
It's far from ideal for your wife, but I hope she manages to find peace and is able to pay her respects in an alternative way.