One of the negative effects would be a $2,500 fine and spending a few weeks or years in prison for not having a working catalytic converter as required by federal law. Your friends' dad would also get to share in the fun to the tune of $25,000 and even more time in prison for being the agent who intentionally did such a thing.
One of the other negative effects would be the poisonous carbon monoxide you'd have coming out of the exhaust, that you'd wind up breathing anytime the vehicle is idling with the windows down, or that other people on the road next to you in traffic would be breathing. Of course, if you smoke, this might be a positive, you could just suck on the exhaust pipe with the vehicle running and it'd be like a giant cigarette.
Generally you're better off having a new replacement converter put in, either by buying a new pipe with the converter already on there, or buying an aftermarket converter and putting it in place of the stock converter.
You can also have the shop do some custom exhaust work while you're at it and get better performance while keeping it fully legal.
Unless it's an off-road only vehicle, federal law requires a working catalytic converter. There are yahoos who use straight pipes or fake converters to try and skirt the rules and they might get away with it for a little while, but the big ticket and the time is a wake up call when they do get it.
Don't try it, spend the money and replace the converter if that's what it needs. Buy a universal converter/converters online if you need to and have a shop install them if that costs less than buying them at a shop. Even if it costs a few hundred, it's cheaper than $2,500 and years of your life.
