On this I believe the husband's first priority is to his existing wife. This is very difficult for her, and it is his job to lovingly care for her. He probably also promised on their wedding day to have her "forsaking all others", a promise that she most likely felt was the most important part of his vows on the day, and not something she can put aside lightly. His task now is to lovingly attempt to bring her around to the idea.
There may be an occasion in the future when, if she continues to put her foot down, but God is clearly saying something else, that the husband must make the difficult choice to follow what he believes God is saying despite the objections of his wife. But this should be an absolute last resort, as it isn't going to make for a harmonious marriage...
To put this into perspective, in Muslim countries I understand the law tends to be that the consent of the first wife is required in order for a man to take a second wife, apart from in a few specific circumstances when her wishes can be overruled (such as if the first wife is infertile and the man wishes to have children by another). We don't follow Islam, but rather follow the Bible, and this law does have the perverse result that some Muslim men take second wives in secret to avoid getting consent from their first. However if even the Muslims are able to officially respect the wishes of the first wife so highly, only overruling her when there are strong mitigating circumstances, surely we can respect her in the same way.
