Eyes provide a very significant benefit for survival, and contrary to popular religious arguments, at their most basic are fairly easy to develop naturally, and once they develop naturally, natural selection will cause improvements. Because of that, it is not really surprising that they have evolved independently multiple times.
If you would like a better understanding of why this is true, I very highly recommend the book Why Evolution is True by Jerry Coyne.
(Sorry, I wrote this reply before realizing what sub I was in... but the question seems legit, so I decided to post the answer anyway)
Most of my in-depth knowledge came from university courses, but there are lots of good books out there. I've read this one and would definitely recommend it, it doesn't require any prior knowledge of the topic.
my advice is to buy your mother-in-law one or all of these books:
• jason rosenhouse's *among the creationists *
• richard dawkin's *the greatest show on earth *
• jerry a. coyne's *why evolution is true *
• bill nye's *undeniable: evolution and the science of creation *
edit: a word.