People realize that whoever gets elected into office will in some way take advice from their spouse during their time in office. If there is a disagreement between spouses on a major issue it could cause worry from voters that the issue will not adequately be addressed by the candidate. This issue is a big deal during this campaign, in regards to immigrants and citizenship, so it is important for Donald and Melania to tackle this issue as a unified front.
Although a somewhat embarrassing gaff, I do not believe that Melania Trump misstating Donald Trump's ban on Muslims will be harmful to Donald's campaign. Even if nothing else his current campaign success seems to have succeeded in breaking all the established rules for what will or will not turn voters away from him - in this case any publicity seems to be good publicity.
Although Melania Trump misstated her husband's ban on Muslims, it is not likely to hurt his presidential campaign. Many of his supporters believe Muslims should be banned from the United States. They watch the news and think all who are Muslims believe in terrorism, despite this not being the case.
Donald Trump won't be taken down by his wife. Donald Trump is a master at manipulating the media. He knows exactly what to say and when to say it. He has more time in front of the camera than anybody. The only way he loses is if the voters rally behind only one of his opponents.