It would depend on the friend. I have a good friend who is the world's worst speller. Also, she can't place an apostrophe correctly to save her life. She's been like that since high school, so unless it's an especially egregious error (like, you can't make out her meaning) I don't usually call her out.
On the other hand, Know-It-All Sister was an editor on her high school paper, and a Journalism major in university. If she makes a mistake, I love ragging on her about it. And also, 'cuz she's my sister.
I did actually say something to a fellow OSer on facebook. But only because her spelling error conjured up a really funny vision. She meant to say 'desert rats', but accidentally added an 's', turning it into 'dessert rats' which made me picture rodents with a dollop of whipped cream on their heads topped off with a maraschino cherry.
I usually just let it go on facebook but the text speak makes me mental - I know people are typing on their phones and all but...