In cali its definitely a big no no, as long as you dont get pulled over. And it doesnt matter if you have a good driving record...mine is clean and I still get pulled over (maybe because I'm Asian?who knows. Oh well right? Cant do nothing about it..they can mess with you no matter what as long as your car is fixed up.) Main thing is if your friend is getting his car smog there should be a problem..all they usually check is gas pressure, aftermaket headers or air filter. Usually if we have an air filter we can still pass.