What Google hates?
1. Serving different results for search engines and users.
2. Content copied from different sites.
3. Buying paid links to manipulate site ranking.
4. Keyword stuffing in content and HTML tags.
5. Hiding keywords using style sheet.
7. Slow loading sites.
8. Low server response sites.
9. Duplicate URLs and sites that don’t use canonical URL tags.
10. Dynamic sites that doesn’t use parameter handling.
11. Pages that have high bounce rate.
12. Tags that doesn’t meet body content.
13. Low average time spent on each page.
14. Non-search engine friendly URLs
15. Images that are not optimized which results in slow loading pages.
16. Interlinking with your own hosted domains.
17. Getting traffic from Spyware or through running some sort of scripts.
18. Links to spammer sites and bad neighborhood sites.
19. Having same content in your domain and sub domains.
20. Pages that installs Badware.
21. Affiliate program with no unique or relevant content.
22. Too many reciprocal links with irrelevant content sites or reciprocating links with bad neighborhood sites.
23. Misspelled words to get higher ranking in search results.
If you find any of these Google hates (Black hat SEO techniques) in your site or your friend’s site, take immediate steps to resolve or the site will get banned at any time or will be flagged as spam in search. If the site got banned for not following Google guidelines, take immediate measures by analyzing the above mentioned points and fix accordingly. Once you're are sure that you follow the Google guidelines then you can use the Google index reconsideration form.