Shouldn't there be a comma before that last "And"? Well, unless you're one of those people that like to leave out commas after the second to last item in a series.
Nope. Let's take a look:

"<i>I have a friend who is dyslexic, I don't hold it against them, but dylexia isn't that common and people with horrible grammar can't just hide behind something like that.</i>"

Now, there should be some sort of full-stop after "dyslexic." A semi-colon would do - but that's the only error. "I have a friend who is dyslexic," "I don't hold it against them," "dyslexia isn't that common and people with...like that" are three indepedent thoughts. The comma is, and belonds before the "but dyslexia..." but is superfluous and wrong before the "and," as it's only a series of two. Unnecessary commas are just as bad as ones not used in the first place.

Ah! Americanisms! No commas anywhere near 'and(s)'. You would get completely owned for that in the UK.
Commas are often absolutely necessary before "and," such as when used to combine two independent clauses. However, when used in a series "I like this, this, and this" apparently now it's proper grammar whether you include the last comma or not. This is just another example of how English has been perverted to comform to lazy people. As if this wasn't bad enough, now it's acceptable to use split infinitives! The grammar world is truly going to quickly go hell in a handbasket. *rolls eyes*