First off, neither Coco nor The Book of Life have anything to do with Mexican Catholicism. They are amusing animation movies. Why the heck would your friend think otherwise? Did he expect The Nightmare Before Christmas to be a historical presentation about All Saints’ Eve and St. Nicholas of Myra?
Second, if you want to understand the All Souls’ Day/Day of the Dead customs in Mexico, you want to look at Spanish and colonial Catholicism during the 1600’s, 1700’s, etc. Very little of this stuff is of native origin; it just has special Mexican expressions.
Candy skulls and skeletons are the same kind of “memento mori” imagery that is found all over Europe, particularly in medieval “dance of death” paintings in churches and cemeteries. The main message is a reminder that you are going to die, and that you should look after your soul. The Day of the Dead message is that you should pray for the holy souls in the Purgatory part of Heaven, and that you should remember with gratitude that they are praying for you.
Eating and distributing candy and goodies is also meant to remind people to pray for the dead; it’s a teaching tool for kids, particularly. In Germany, it’s the job of Catholic godparents.
Taking picnics to cemeteries on All Souls’ Day is a way of socializing with one’s dead family that goes back to pagan Roman times. (Although the Church outlawed a few features of such meals.) It is found all over Europe. Germans do weird things like taking giant stick candy and setting them up in front of graves like flowers, or making giant pretzels on sticks for the same thing. Countries differ on how much humor and whimsy is used (to cheer people up who are grieving), vs. how much serious and dark decoration and food is done.
Since Mexico is a warmer country, all night vigils of prayer at the cemetery or church are more popular – but even in cold countries like Poland, people have been known to do it.
Same thing with making temporary altars and decorations, which are associated in Europe and the Americas with all sorts of processions and festivals (including All Souls’ Day).
So yes, Coco and The Book of Life are basically the same sort of thing as Tolkien’s “Leaf by Niggle.” It’s just that Tolkien knew what he was writing, and Hollywood didn’t.