Good advice above, plus:
1. You will violate many different ToS by doing so
2. It is incredibly annoying - we get thousands of spam emails a day to our catch-all WHOIS email address (we own hundreds of domains)
3. Depending on how you go about querying the WHOIS database, you will encounter captcha checks, query frequency limits and / or query count limits
Having said that though, I think your client would bear the brunt of any repercussions from such an endeavor, so if you are morally and ethically alright with it, code it up and make sure not to put your name on it...just skip our URL when you come to it
