I think sometimes Mexico is a friend and other times not much of a friend. It depends on the issue.
There are many people in the Mexican government who perpetuate the idea that we 'Americans' stole Texas and other southwestern U.S. territory from them. (Technically Mexicans are Americans also.) As a result some of the people who come here believe they have every right to be here illegally or otherwise.
This is a major reason Mexicans (and even natural born Mexican-U.S. citizens) disrespect our flag and prefer to fly their own. They really believe we stole major parts of the U.S. from them and that those sections of the U.S. really belong to them. So they prefer to dis our flag and our authority.
The cost of building a China-like 'Great Wall' as Mr. Trump seems to envision, would be prohibitive. With no middle class left and only the working class paying taxes since the wealthy are given a pass on that, we would have to scrap some major programs in order to finance such a wall.
After we've dug up the money for the wall, a major undertaking by itself, we would need to actually build the wall. Since Mr. Trump says he will force the Mexican government to pay for the wall, will he, true to his own character, also hire Mexicans and other foreigners to build it? Regardless of who builds it, it will take a very long time to accomplish. Not likely you or I -- or Mr. Trump, would see the completion of such a project.
The only way any wall might do what it is supposedly designed to do, keep Mexicans and other illegals out, would be to station guards (or drones) every 20 feet or so with orders to shoot to kill anyone trying to get through from the Mexican side.
While Trump might welcome such an idea, actually carrying it out is another matter. The military would have to be increased considerably as it is already spread pretty thin with operations already on our plate. We have a long border with Mexico and placing an armed guard every 20 feet of it would be quite a task.
Mexico actually encourages it's citizens to come here from what I've heard. So in that sense Mexico is not our friend. It is promoting untruths about who our country really belongs to and it encourages its citizens to break our immigration laws.
A wall is not practical for many reasons, and Mexico is a sometimes friend.