If you search back through threads on this site, you'll see that I'm in the same position that your friend is. I'm also going to assume you mean a C6 Z06 vs a gen 3 or gen 4 viper, since a C5 Z06 is now only 20k or so and a comparable year viper costs much more.
I compared a gen 3 viper and a c6z as they are both in the same price range. However, I realized after experiencing both that they are not as similar as you would think. I grew up dreaming about a Corvette as my future sports car, and vipers as an impossible dream. Partly because of their exotic rarity and partly because of all the complaining usual car magazines and shows did about the car. The viper was always compared to vettes and no matter which model was faster than the other, the reviewers almost always chose the vette.
So I started saving up for a c6z and I didn't pay any serious attention to vipers. I read up on everything z06 related and decided what years, colors, etc I wanted. Then, I decided that before I purchase one, I should experience one just to be sure. I also decided I should have something to compare the car to, so I chose my "foolish childhood dream" - the viper.
I found the vette to be everything I had expected...it was great. Great sounds, great practicality, great noise, etc. I was disappointed there wasn't a convertible option, for the z06, but I knew I would be happy with a fixed roof anyway.
Then I posted my thread on here looking for a test drive...instead I got a ride in one from a member here, and everything I had planned with a vette melted away. The viper was about a hundred times more exciting than the vette. Not only that, but every complaint I read or heard about the viper turned out to be a non-issue for me. I finally drove one a couple weeks ago, and that sealed the deal. It was a rough, no compromise car and I loved it.
So what do I recommend? Your friend needs to try both out for themselves. I think most people would choose the z06. I know some viper owners won't agree, but I'd say the z06 beats the viper in almost every category on paper. However, for me there became no question as to what I would own. The viper has the advantage in all the intangibles. Things you can't read about or have someone describe to you. You just have to experience it yourself. That's why I chose a viper, and why I'm buying one this winter.
Good luck to your friend.