There are cases in storytelling where a character will have specific speech patterns that make it okay to use phrases that you wouldn't normally hear in spoken conversation (off the top of my head, the Aiel characters might be an example of this), but Perrin isn't one of those cases. And Sanderson's dialogue has always been more functional than smooth and realistic, so I can only imagine it gets worse when he's working with characters he didn't create in the first place.

Though, Sanderson's dialogue in his most recent Mistborn book (The Alloy of Law) was a definite step up, and even The Way of Kings showed an improvement over the original Mistborn trilogy on the dialogue scale, so I have to think he's getting better at it. It just obviously hasn't translated into WoT, working with someone else's characters and not writing completely in his own style.

There's also the fact that Perrin and Rand haven't been together in more than seven books, if I remember, and when they last parted Rand was pretending to be pissed off at him, so maybe one "my friend" is justified if you can find the right place for it. But not three.