NO GUTTING CATS IS BAD!!!!!!!!! He just lot power and tq. Your friend now has what is called flow stagnation. The cat acts like a box which traps exhaust and interupts the flow. Heres a picture of what your friend just did.
So since he has screwed those up For your friends first mods he needs to go get new cats. Id recommend high flow cats, but if you must straight pipes work just fine.
Sorry to seem like an ass but I have heard of too many people doing this and getting the misconception that it good.