It is anecdotes like this that lead me to believe that Western society in general and the US in particular are not going to survive as we know them much longer.
For a society to survive and thrive the vast majority of people must agree on certain facts. The Axis powers could not have been defeated if half the population did not think their existence constituted an existential threat to Western civilization.
Your cousin is being educated at one of the most highly respected universities in the world. Her belief system is so strong that it does not allow her to accept facts that are contrary to said belief system. The existence of MS-13 in the US constitutes an existential threat to anyone who comes in contact with them.
We are in deep trouble.
For about seven decades lots of people denied that the Mafia existed.