Mexico would have to take Texas and part of California, and we know that isn't going to happen. The Treaty of Guadalupe Hidalgo remains in force, having been signed in 1848. It ended the Mexican-American War and ceded land to the U.S. in exchange for millions of dollars paid to Mexican citizens as well as the U.S. assuming responsibility for debts Mexico owed. There is no evidence to suggest that Mexico intends to break the treaty.
The relationship between the United States and Mexico benefits Mexico much more than it does the United States. Even though some politicians may not like Donald Trump, they will not jeopardize the jobs and other financial benefits of being allies with the United States if Trump becomes president. The want to get along.
If Donald Trump is elected president, Mexico will not claim its former lands lost to the United States during the Mexican War. These lands belong to the United States and make up several Southwest states. Mexico has no way to reclaim these lands; any attempt to do so would be a catastrophe for the country.
No, Mexico will not claim the United States took land from them, but they also will not respect Donald Trump as president. He has insulted the Mexican people by calling them rapists and drug lords; why should they meet with him? Our relationship with Mexico will deteriorate with time if Trump is president.