Well i am a web designer & was also worked as seo for basic on page & off page, i know the back links are most important for getting rank of site, but i have a secret question regarding On page.

I have a friend who also going SEO work as freelancer, he said! he doing only strong On page for site & not doing any off page & link building strategy for any website, also for testing him i gave him a client site for ranking & yes within 2 weeks he get rank of that site for 4 competitors keywords, which still on top page in three main search engines. I was asked him about his on page strategy but he refuse me & did not told me about anything.

So here if anyone knows about this than please let me know i have a client & i need to get rank of site because we agreed.
Please share your suggestions.
Thanks